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

[via llvm-commits]

whitequark wrote:

> 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).

`LLVM_ON_UNIX` is definitely a mixed bag for WASI and I'll take another look at whether it should be defined or not when I rework the patch to use `defined(__wasi__)` instead.

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