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

[A. Jiang via llvm-commits]

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+// -*- C++ -*-
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frederick-vs-ja wrote:

When I attempted to just remove `stdaligh.h`, there was build error. Perhaps CMakelist.txt should also be modified.

I'm not sure whether we can/should just be inconsistent for `stdaligh.h`, given libc++ is providing almost all wrapper C headers required by the C++ standard library.

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