Replaces PR #372.

Build: Add g++/clang warnings to match some enabled by /W4 in MSVC

HLSL: Add texture Load method & decomposition

AMD: Implement 4 AMD-specific extensions.

HLSL: Add SampleCmp and SampleCmpLevelZero texture methods

Also, the per-build part had stopped auto-updating,
which is fixed locally as a git hook.

- Support GL_AMD_shader_ballot (SPV_AMD_shader_ballot).
- Support GL_AMD_shader_trinary_minmax (SPV_AMD_shader_trinary_minmax).
- Support GL_AMD_shader_explicit_vertex_parameter
  (SPV_AMD_shader_explicit_vertex_parameter).
- Support GL_AMD_gcn_shader (SPV_AMD_gcn_shader).

Protect more against error recovery of bad built-in variable redeclarations.

HLSL: Add tx.GetDimensions method (uint returns only)

HLSL: Add SampleBias and SampleGrad methods, + associated tests

Note: Technically, this is glslang's modified private copy of official headers.
Official headers will appear in other places, like
https://github.com/KhronosGroup/SPIRV-Headers.

Build: Reorder fields to avoid compiler initialization order error.

HLSL: add in/out/inout qualifiers

HLSL: add offset Sample() form and arrayed texture support

HLSL: Sampler/texture declarations, method syntax, partial Sample method

Added -C option to request cascading errors.  By default, will exit early,
to avoid all error-recovery-based crashes.

This works by simulating end-of-file in input on first error, so no
need for exception handling, or stack unwinding, or any complex error
checking/handling to get out of the stack.

Added -x option to save SPIR-V as 32-bit hexadecimal numbers to a text file.

Includes adding test cases to verify the differences.

This is used by OpenGL, but not Vulkan.
Includes:
 - atomicCounter, atomicIncrement, atomicCounterDecrement
 - atomic_uint layout-offset checking
 - AtomicStorage capability