[libcxx] [llvm] [libc++] Deprecate and remove meaningless `<cxxx>` headers (PR #111615)

[Jonathan Wakely via llvm-commits]

jwakely wrote:

I don't think we'll ever physically remove these from libstdc++, certainly not as a DR. But I should add `#warning` to them saying they're deprecated/removed.

https://github.com/llvm/llvm-project/pull/111615