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
Chris Bieneman