[mlir] [openmp] [llvm] [clang] [lld] [libcxx] [libc++] Deprecate the _LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS macro (PR #77692)

[Mark de Wever via cfe-commits]

https://github.com/mordante approved this pull request.

LGTM modulo one nit and when the CI is green.

https://github.com/llvm/llvm-project/pull/77692