[clang] [Clang][HLSL] Add environment parameter to availability attribute (PR #89809)

[Helena Kotas via cfe-commits]

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hekota wrote:

Right, and I am adding a lot more HLSL-specific tests in my upcoming PRs that implement the diagnostic modes under the SemaHLSL directory. There is a number of platform-specific tests for the availability attribute already here under Sema though so I figured we should have some basic coverage here as well.

https://github.com/llvm/llvm-project/pull/89809