This PR significantly reworks the way glslang is versioned.

Instead of committing changes to the `GLSLANG_MINOR_VERSION` define in
`glslang/Public/ShaderLang.h`, and using `make-revision` to generate
`GLSLANG_PATCH_LEVEL` in `glslang/Include/revision.h`, all version
information is now derived from the new `CHANGES.md` file.

`CHANGES.md` acts as the single source of truth for glslang version
information, along with a convenient place to put all release notes for
each notable change made.

`CHANGES.md` is parsed using the new `build_info.py` python script.
This script can read basic template files to produce new source files,
which it does to read the new `build_info.h.tmpl` to generate (at build
time) a glslang private header at
`<build-dir>/include/glslang/build_info.h`.
I've written generators for each of the CMake, Bazel, gn, and
`Android.mk` build scripts.

The new version code conforms to the Semantic Versioning 2.0 spec.

This new version is also used by the CMake rules to produce versioned
shared objects, including a major-versioned SONAME.

New APIs:
---------

* `glslang::GetVersion()` returns a `Version` struct with the version
  major, minor, patch and flavor.

Breaking API changes:
---------------------

* The public defines `GLSLANG_MINOR_VERSION` and `GLSLANG_PATCH_LEVEL`
  have been entirely removed.
* `glslang/Public/ShaderLang.h` and `glslang/Include/revision.h` have
  been deleted.
* Instead, `<build-dir>/include/glslang/build_info.h` is created in
  the build directory, and `<build-dir>/include` is a CMake `PUBLIC`
  (dependee-inherited) include directory for the glslang targets.
* `<build-dir>/include/glslang/build_info.h` contains the following
   new #defines:
   `GLSLANG_VERSION_MAJOR`, `GLSLANG_VERSION_MINOR`,
   `GLSLANG_VERSION_PATCH`, `GLSLANG_VERSION_FLAVOR`,
   `GLSLANG_VERSION_GREATER_THAN(major, minor, patch)`,
   `GLSLANG_VERSION_GREATER_OR_EQUAL_TO(major, minor, patch)`,
   `GLSLANG_VERSION_LESS_THAN(major, minor, patch)`,
   `GLSLANG_VERSION_LESS_OR_EQUAL_TO(major, minor, patch)`
*  The CMake install output directory contains a copy of
   `build_info.h` at: `include/glslang/build_info.h`
*  Python3 is now always required to build glslang (likely always
   required for transitive dependency builds).

Use CMake's builtin functionality for pre-compiled headers

Make sure glslang_angle has a definition in BUILD.gn

Set the value to false if the environment doesn't declare this variable.

`glslang_pch()` did manual mangling of the compiler flags to enable pre-compiled headers.
I couldn't get this approach to work with the `MachineIndependent` subdirectory, but fortunately CMake has added first-class support for precompiled headers in 3.16, which does work with subdirectories.

Moved `glslang_pch()` to the other global function declarations.

`glslang_pch()` is a no-op when using CMake earlier than `3.16`.

CMake's PCH implementation does not need the `pch.cpp` files, so just remove them.

Remove GLSLANG_BUILD_PIC flag

Add new rules to .gitignore

On closer inspection, it appears that nearly all the targets use the `POSITION_INDEPENDENT_CODE` target option anyway.
Simplify all this away by always being PIC.

`GenericCodeGen` and `MachineIndependent` were missing from the generated visual studio solutions. Add these.

`.vscode/` ignores Visual Studio Code user config files
`bazel-*` ignores bazel build system symlinks.
`out/` ignores the default output directory for Visual Studio generated files.

gn: Fix dawn tests in Chromium

Add --quiet option.

Being quiet should have been the default, but I guess it's too late
now to change it.

Dawn tests use shaderc, which assumes glslang has HLSL support.  This
change makes HLSL support also follow template arguments, and changes
the target names such that glslang_sources will remain the "has all
features" target and the new glslang_lib_sources would be what ANGLE
would use.
Signed-off-by: Shahbaz Youssefi <ShabbyX@gmail.com>

gn: Fix `gn gen --check` by adding missing dependency

Signed-off-by: Shahbaz Youssefi <ShabbyX@gmail.com>

gn: Optionally disable optimizations and HLSL

Add GLSLANG_BUILD_PIC CMake flag

Don't use add_link_options() on old CMake versions

Enables `-fPIC` compiler flag even when building statically.
This is helpful for statically linking a `glslang` target into a shared library.

Simplifies the workarounds seen in google/shaderc#1093 to a `set(GLSLANG_BUILD_PIC 1)`.

To reduce the binary size of ANGLE, a gn override is added
(glslang_angle) which:

- Controls whether ENABLE_OPT=1 is set
- Customizes the build for the Vulkan backend of ANGLE.  As a first
  step, this removes HLSL functionality which together with no
  optimization shave ~2.5MB off of ANGLE's binary size.

Upcoming changes will add a macro for GLSLANG_ANGLE similar to
GLSLANG_WEB that will strip features from glslang to support only what
ANGLE needs.
Signed-off-by: Shahbaz Youssefi <ShabbyX@gmail.com>

Fixes: #2315

Kokoro: Correct the `build_file' path to build.sh

License headers: s/Google/The Khronos Group

This was a copy-paste screwup, where the first line of the copyright had the company name was updated, but the company name mid way though was not.

Add license checking continuous and presubmit tests

Fix GLSLANG_IS_SHARED_LIBRARY define

The `license-checker` is a tool that verifies each file has contains a permitted license header.

See https://github.com/ben-clayton/license-checker for more information.

It was incorrectly always being set, causing linker warnings for MSVC builds.
Also simplify the preprocessor nesting in `glslang\Public\ShaderLang.h`

Add copyright headers to build files and scripts.
Simplifies automated scanning for bad license headers.

HLSL: Fix #pragma pack_matrix(row_major) not work on global uniforms

glslang: Only export public interface for SOs

Location validation, with update to SPIR-V tools validating location

This introduces five new "Validation failures":
- baseResults/hlsl.semantic.vert: issue with gl_ClipDistance/CullDistance
- baseResults/spv.430.vert: issue gl_ClipDistance
- baseResults/spv.450.tesc: still unknown
- baseResults/spv.dataOut.frag: gl_FragData should not be supported, problem with front end
- baseResults/spv.meshShaderPerViewUserDefined.mesh: seems okay, maybe a problem with SPIRV-Tools

This adds or changes binding/location decorations in 100s of shaders.
It also allows more output (spv.register.autoassign.rangetest.frag)
due to allowing ioMap() to fail.

Add additional licenses in use to LICENSE.txt

Ideally we'd unify the licenses in use by changing the licenses in the file headers to BSD-3-clause.

Until then, let's correctly list all the licenses currently in use.

Issue: #2305

Related to https://github.com/KhronosGroup/SPIRV-Cross/issues/1414.
The real problem is either using DX10 semantics for DX9 or missing
functionality in DX10 parsing.