[clang] [HLSL] Add StructuredBuffer to external sema source (PR #106316)

[Damyan Pepper via cfe-commits]

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

I had a bunch of questions about the tests, but I see that what's happening here is that the same tests for RWBuffer are being adapter for StructuredBuffer.

I don't know how much everything differs between the two...I'm wondering if there's some way to have a single place where we say "here's the stuff that should work for all buffers" (maybe sliced further based on other properties) so we can be sure that we have a consistent set of tests across them.

Maybe not something to resolve in this PR, but might be worth thinking about (or maybe deliberately rejecting).

https://github.com/llvm/llvm-project/pull/106316