[clang] [NFC] Add API documentation and annotations (PR #78635)

[Chris B via cfe-commits]

llvm-beanz wrote:

@farzonl I like that idea, but I think there's a lot more work to do in order to get proper HLSL documentation coming out here.

For these files should get processed as part of Clang's normal doxygen build, so they should get pulled in with the other clang API docs.

https://github.com/llvm/llvm-project/pull/78635