[clang] [llvm] Conditionalize use of POSIX features missing on WASI/WebAssembly (PR #92677)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

Ah thank you for pointing that out, I had missed some updates on that thread.

I think `defined(__wasi__)` would address my concerns. @MaskRay asked a good question about whether we should tie this to `LLVM_ON_UNIX` as well and I don't have a strong intuition there, but my naive thinking is that it would help in some cases but be insufficient in others (e.g., within Support/Unix/Path.inc, we'd still need to have a conditional for WASI).

https://github.com/llvm/llvm-project/pull/92677