Infrastructure: Generalize and broaden per-variable extension checking.

Fix a few GN build issues.

More precisely check for the qualifiers that enable the vulkan memory model for buffer references.

Not all memory qualifiers necessarily need the vulkan memory model, e.g. volatile/restrict that correspond to core SPIR-V features do not.

 * adds a source set for default resource limits to mirror CMake
 * adds a target executable for the standalone validator
 * fixes a missing warning

Allows ANGLE to use the integrated BUILD.gn instead of a custom one.

ANGLE bug: 3088

Fix xfb_stride incorrectness(#1654)

HLSL: Avoid leaking a shadow sampler struct

Avoid leaking name for counter buffer

HLSL: Fix #1655; use "" for nullptr file names

Add int64 support in XFB. Change containsDouble to contains64BitType. Make
it more general.

Work around a CMake cross-compilation issue

Update SPIRV-Tools known good: relax Uconvert check

Permit UConvert as an OpSpecConstantOp operation when
SPV_AMD_gpu_shader_int_16 is used.

Parenthesize to avoid GCC warning

Don't output duplicate OpTypeAccelerationStructureNV declarations

GL_EXT_buffer_reference

* Make sure source strings are terminated

The source strings may or may not have a null terminator. We need to
make sure we add one before outputting the source strings as we iterate
over the c-str looking for the null terminator.

* Review feedback

Mark spv.int16.amd.frag.out as failing validation

See #1642

Also updates SPIRV-Tools and SPIRV-Headers known_good.

CMake can't correctly configure the latest code because of an issue with
cross-compilation of pthreads:
https://gitlab.kitware.com/cmake/cmake/issues/16920

Create separate OpSource for each included file when compiling -g

Add passes to spirv-opt passes to preserve source line info when compiling -g.

CMake: Fix linking pthread of CMake < 3.1

As reported in #1624, Ubuntu 14.04 LTS still uses CMake 2.8.12 which
does not support the Threads::Threads target (added in CMake 3.1).

This could be reverted once the required CMake version is bumped to 3.1+.

Move memory model scope tests to Vulkan 1.1 batch

Fixes #1610

* Updated expected result due to version change

Fixes #1630 and fixes #1631.