This needs some render testing, but is destined to be part of master.

This also leads to a variety of other simplifications.
 - IO are global symbols, so only need one list of linkage nodes (deferred)
 - no longer need parse-context-wide 'inEntryPoint' state, entry-point is localized
 - several parts of splitting/flattening are now localized

SPV: Emit names of specialization constants, fixes #710

Install headers

This enables the vulkan loader to be built against an installed glslang.

CMake: add target_link_libraries internally

SpvBuilder: add const specifier to vector reference arguments

Use correct OriginUpperLeft when linking more than one shader.

Semantics: Texture clamp functions are available for fragment shader only

Fix a CMakeLists issue

Fix build if NV_EXTENSIONS is not set

support SPV_NV_viewport_array2 and SPV_NV_stereo_view_rendering

Makes it easier to include glslang in a larger CMake project---instead
of having to call `target_link_libraries(glslang OSDependent OGLCompiler
HLSL)`, for example, you only need to call
`target_link_libraries(glslang)` and it will pull in the helpers it
needs.

This is also better in terms of cleaning up the "public interface",
of sorts, for building glslang: end-users probably shouldn't need to
know or be explicitly dependent on internal targets.

Fix for not-handled-in-switch warnings

Added default to the switch() for AMD_EXTENSIONS to avoid a spew
of warning messages.

Fix compilation with MSVC 2010

HLSL: fix dereferencing when copying split structures with arrays

Implement SPV_KHR_subgroup_vote

SPV: Implement new revision of extension GL_AMD_shader_ballot

When copying split types with mixtures of user variables and buitins,
where the builtins are extracted, there is a parallel structures traversal.
The traversal was not obtaining the derefenced types in the array case.

- Add support for invocation functions with "InclusiveScan" and
  "ExclusiveScan" modes.
- Add support for invocation functions taking int64/uint64/doube/float16
  as inout data types.

doc.cpp: Add capabilities, scope to the opcodes. Add opcode and
capability strings.
GLSL.ext.KHR.h: Add extension
string.
GlslangToSpv.cpp: Fix handling of opcodes to generate
appropriate SPIR-V.
spirv.hpp: Add capability and opcode
enums.
spv.shaderGroupVote.comp.out: Update SPIR-V output for test
shader.

This reduces code duplication in a few ways, and better encapsulates
vector swizzle representation.

HLSL: Improved overload handling