[clang] [llvm] [HLSL] Add handling of Interpolation Modifiers (PR #225452)

[Deric C. via llvm-commits]

https://github.com/Icohedron approved this pull request.

LGTM. Nothing I can see wrong with the changes in this PR in particular. 
Just update the [original wg-hlsl proposal](https://github.com/llvm/wg-hlsl/blob/a7681e78c9219ee05170b3ea97e4dc9e6a12ca51/proposals/0047-semantic-signatures-metadata-schema.md) so that it is more precise and reflects the actual implementation.

https://github.com/llvm/llvm-project/pull/225452