[HLSL] Initial codegen for SV_GroupIndex

Semantic parameters aren't passed as actual parameters, instead they are
populated from intrinsics which are generally lowered to reads from
dedicated hardware registers.

This change modifies clang CodeGen to emit the intrinsic calls and
populate the parameter's LValue with the result of the intrinsic call
for SV_GroupIndex.

The result of this is to make the actual passed argument ignored, which
will make it easy to clean up later in an IR pass.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D131203