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

[Luca Versari via cfe-commits]

veluca93 wrote:

FYI I recently rebased this PR on a more recent LLVM (see https://github.com/veluca93/llvm-project/commit/a2a65fd61594a3fdbc2afed8bfe3fa4dcdafff08), although I believe llvm has changed enough that some more changes are needed after this (I ultimately ended up adapting my old patches: https://github.com/veluca93/llvm-project/commit/79b75612b0aec63277ad748d8e305bf507d0224a and https://github.com/veluca93/llvm-project/commit/f7da254c59030b6e72857140a46333d9ce156eb2)

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