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

[James Y Knight via cfe-commits]

jyknight wrote:

My suggestion on #69994 had been to stop implying `_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS` from `_LIBCPP_ENABLE_CXX20_REMOVED_FEATURES` in LLVM 18 at the same time as deprecating it. Did you intend to _not_ do that, or was it just missed?

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