[HLSL] Fix MSFT Attribute parsing, add numthreads

HLSL uses Microsoft-style attributes `[attr]`, which clang mostly
ignores. For HLSL we need to handle known Microsoft attributes, and to
maintain C/C++ as-is we ignore unknown attributes.

To utilize this new code path, this change adds the HLSL `numthreads`
attribute.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D122627