[clang] [HLSL] Add Append/ConsumeStructuredBuffer definitions to HLSLExternalSemaSource (PR #113643)

[Justin Bogner via cfe-commits]

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

Looks good. I have a comment about the testing strategy in general but that can be addressed separately from this PR.

https://github.com/llvm/llvm-project/pull/113643