Latest known-good SPIRV-Tools: WARNING: Needs python 3.x.

You may need to install python 3.x and also
[windows users] may need to get it in your path in front of python 2.7.x.

If you don't need the external projects (i.e., SPIRV-Tools) and don't want
to upgrade to python 3, don't create the External subdirectory and CMake
won't require python 3.  (CMake has a bug that makes it find the wrong one
on a first pass if the top level does not require the version needed by
lower levels.)

For nonuniformEXT constructor, make a copy of the node to decorate

Add support for GL_EXT_buffer_reference2

Fix issues of explicit conversions.

If a shader's only use of float16 is for FConvert, add the Float16 capability

When visiting instructions, check that there are no 16-bit storage capabilities
and add the Float16 capability. Same for int8/int16.

For certain int8/int16/float16 conversions,they are conditionally allowed
when extensions defining explicit arithmetic types are enabled.

This was added some time ago, but I suspect largely unused.
Christoph pointed out of few contradictions to actual convention,
so that's fixed. But, I suspect it is largely incomplete.

introduce ability to dump builtin symbol table

add corresponding EShMsgBuiltinSymbolTable
TSymbol::dump functions have option to do "complete" print
bugfix in TType::getCompleteString, structure can be null for block

Add cstdlib include

On some platforms some of the includers of this header don't have
'atoi' defined after the change in #1749

Update spirv-tools known-good

Check if _MSC_VER is defined before using.

Fixes compilation errors on some platforms.

build: Don't ship GTEST headers in install target

A project that uses googletest(GTEST) shouldn't include googletest
headers with its artifacts in its install target.  These headers
simply are not needed by the consumers of the install target and
can cause conflicts with other projects that use googletest
themselves and this project's install target.  And they are just not
part of the expected glslang build artifacts.

It is likely that the addition of the googletest headers to the install
target was a simple oversight that happens as a result of adding
googletest as a CMake subdirectory.

For more information on how this causes conflicts with other projects,
please see: https://github.com/KhronosGroup/Vulkan-ValidationLayers/issues/821.

Update spirv-tools known-good

Callables shouldn't have incoming ray flags

Enable GL_KHR_shader_subgroup properly

(and GL_NV_shader_subgroup_partitioned) based on GL/ES version
instead of predicating it on vulkan SPV generation

Also add AST testing.
The glsl.450.subgroup* files are largely the same as the spv.subgroup*
The glsl.es320.subgroup* files are the same as the 450 versions, but modified to be ES compatible.