[clang] [libcxx] [libc++] Don't dispatch find to wmemchr under -fshort-wchar (PR #203621)

[Nikolas Klauser via cfe-commits]

https://github.com/philnik777 requested changes to this pull request.

I continue to think that this is badly motivated and I have no desire to support this extension in libc++. So far I also haven't seen any scenario in which using `-fshort-wchar` is sound.

https://github.com/llvm/llvm-project/pull/203621