SpecializationConstant impl stage 1 - API and feature plumbing #4212

Job	Run time
Linux -> Pre-checks	15s
Linux -> Clang-NO_GLSLANG-NO_HLSL-x64, Release	5m 37s
Linux -> GCC-x64, Debug	12m 29s
Linux -> Clang-x64, Release	12m 55s
Linux -> GCC-x64, Release	10m 22s
Linux -> Clang-x64, Debug	13m 17s
Linux -> Clang-NO_GLSLANG-x64, Debug	8m 13s
	1h 3m 8s

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