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

[Helena Kotas via cfe-commits]

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hekota wrote:

I might be worth merging these tests into one, though I am not sure if we should have one for all buffers. I think it would be better to group them, like structured buffers, raw buffers, textures etc. We are also adding the AST for buffer methods to these tests, which will differ between buffers a lot.

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