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

[Louis Dionne via llvm-commits]

ldionne wrote:

I'm fine with the patch in principle, but we do have a problem that the modules for these now-deprecated headers get built whenever the `std` module is built, which breaks the compilation of anything that builds the `std` module. I pulled this down locally to play around and even making these modules `explicit module ccomplex` doesn't fix the issue, so I'm not certain how to fix this.

@ian-twilightcoder Do you have ideas? Basically, the TLDR is that we have headers that now contain an unconditional `#warning` directive and we'd like these headers not to be pulled into the `std` module's compilation unless they are requested explicitly. Making these headers `textual` solves the problem but that seems like a hack.

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