[libc++] Deprecate the _LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS macro (#77692)

As described in #69994, using the escape hatch makes us non-conforming
in C++20 due to incorrect constexpr-ness. It also leads to bad
diagnostics as reported by #63900. We discussed the issue in the libc++
monthly meeting and we agreed that we should deprecate the macro in LLVM
18, and then remove it in LLVM 19 since it causes too many problems.

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

Fixes #69994
Fixes #63900
Partially addresses #75975