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

This PR deprecates `<ccomplex>`, `<cstdbool>`, `<ctgmath>`, and
`<ciso646>` in C++17 and "removes" them in C++20 by special depr

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

This PR deprecates `<ccomplex>`, `<cstdbool>`, `<ctgmath>`, and
`<ciso646>` in C++17 and "removes" them in C++20 by special deprecation
warnings.

`<cstdalign>` is previously missing. This PR also tries to add them, and
then deprecates and "removes" `<cstdalign>`.

Papers:
- https://wg21.link/P0063R3
- https://wg21.link/P0619R4

Closes #99985.

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Co-authored-by: Louis Dionne <ldionne.2@gmail.com>

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