[HLSL] Add Semantic syntax, and SV_GroupIndex

HLSL has a language feature called Semantics which get attached to
declarations like attributes and are used in a variety of ways.

One example of seman

[HLSL] Add Semantic syntax, and SV_GroupIndex

HLSL has a language feature called Semantics which get attached to
declarations like attributes and are used in a variety of ways.

One example of semantic use is here with the `SV_GroupIndex` semantic
which, when applied to an input for a compute shader is pre-populated
by the driver with a flattened thread index.

Differential Revision: https://reviews.llvm.org/D122699

# Conflicts:
# clang/include/clang/Basic/Attr.td
# clang/include/clang/Basic/AttrDocs.td

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