[libc++] Deprecate removed features macros. (#77879)

We discussed the removal of these enable-all macros in the libc++
monthly meeting and we agreed that we should deprecate these macros in
LLVM 18, and then remove them in LLVM 19 since they can silently enable
deprecated features that are implemented after the first release of the
macro.

This patch does the first part of this -- it deprecates the macro.

Note that the file
  test/libcxx/depr/enable_removed_cpp20_features.compile.pass.cpp
does not exist so this file is not adapted. Since the feature is
deprecated and slated for removal soon the missing test is not
implemented.

Partly addresses: https://github.com/llvm/llvm-project/issues/75976



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