P0619R4: Reviewing Deprecated Facilities of C++17 for C++20

[cjdb]

Link: https://wg21.link/p0619r4

BEGIN-RST-NOTES
Removed headers are still provided as an extension, but with deprecation warnings.
END-RST-NOTES

[llvmbot]

@llvm/issue-subscribers-c-20

Author: Christopher Di Bella (cjdb)

The [status page](https://libcxx.llvm.org/Status/Cxx20.html#note-p0619) notes that

> P0619: Only sections D.8, D.9, D.10 and D.13 are implemented. Sections D.4, D.7, D.11, and D.12 remain undone.

[frederick-vs-ja]

• D.4 remove <ccomplex>, <ciso646>, <cstdalign>, <cstdbool>, and <ctgmath> [libc++] Deprecate and remove meaningless <cxxx> headers #111615
• D.7 remove uncaught_exception [libc++] Deprecate and remove uncaught_exception #101830
• D.11 remove raw_storage_iterator - this was already done in 578d09c (https://reviews.llvm.org/D101730) but the status page wasn't updated correspondingly.
• D.12 remove get_temporary_buffer and return_temporary_buffer [libc++] Remove get_temporary_buffer/return_temporary_buffer #100914

[frederick-vs-ja]

It seems that <cstdalign> is undesired at all per #37759. But why are other deprecated & removed headers provided?