The SPIR-V gen path is now made default.  Compilation through glslang is
still supported for debugging, and is enabled on the GLSL* end2end tests
for smoke testing.  On release builds, glslang is not supported.

To test with glslang, add the following gn arg (only necessary if dcheck
is disabled):

    angle_enable_spirv_gen_through_glslang = true

Then enable the generateSPIRVThroughGlslang feature.  This can be done
by setting an environment variable:

    ANGLE_FEATURE_OVERRIDES_ENABLED=generateSPIRVThroughGlslang ./angle_deqp_gles2_tests

Binary size saving:

- 1.3MB on Linux (SPIR-V gen itself: 240KB)
- 730KB on Android (SPIR-V gen itself: 140KB)

Perf tests:

- LinkProgramBenchmark.Run/vulkan_compile_single_thread
  * Through glslang:
      truncated mean: 1287033.36
  * Direct SPIR-V Gen:
      truncated mean: 244495.91  (~80% reduction)

- LinkProgramBenchmark.Run/vulkan_compile_multi_thread
  * Through glslang:
      truncated mean: 4565894.83
  * Direct SPIR-V Gen:
      truncated mean: 1158164.10  (~75% reduction)

Bug: angleproject:4889
Bug: angleproject:6210
Change-Id: I486342702977c8114e90073b97183aba115a8b2d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3115140


Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>