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

[via llvm-commits]

whitequark wrote:

> Comments in the config file isn't a harmful thing, but I'm also worried about folks who never see the config file. e.g., they're working in Path.inc on something, see `HAVE_PWD_H` in the code, and just make assumptions from there.
> 
> So perhaps a two-pronged approach: a comment in config.h for the cases where we don't expect a graceful fallback, and a similar comment around the guarded #include in source for the folks who won't look in config.h?

I could do that. What do you think of the fairly strongly worded objection in the RFC thread that requested I use `defined(__wasi__)` instead of individual conditionals?

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