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

[Cyndy Ishida via cfe-commits]

cyndyishida wrote:

> Overall I really like the direction of this. I'm curious if any of the maintainers from Apple have thoughts since they're the primary users of availability annotations.

While I like the approach of aligning availability parameters closer to `llvm::Triple`, I am concerned about how this will interact with existing precedent. There is a lot of code that passes in _platform_ values that aren't actually `OSType`. For example `__attribute__((availability(macCatalyst, ...))` aligns to `EnviornmentType::MacABI` and 
`__attribute__((availability(macOSApplicationExtension, ...)))` aligns to  cli args `-mtargetos=macos -fapplication-extension`. It may become difficult to maintain the distinction between what is accepted as a `platform` vs a `environment`. And we can't realistically re-write client code relying on this. This attribute behavior also aligns very closely with Swift's `@available`
Would it be possible to similarly treat the target shader stages & combinations as platform values as well? Or define a new attribute that does not conflict. 


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