[clang] [Headers] Don't declare unreachable() from stddef.h in C++ (PR #86748)

[Aaron Ballman via cfe-commits]

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

Marking as requesting changes so this isn't accidentally landed when there's an open question as to why it's needed in the first place.

https://github.com/llvm/llvm-project/pull/86748