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

[Nikolas Klauser via cfe-commits]

philnik777 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?

We've also deprecated (or plan to?) `_LIBCPP_ENABLE_CXX20_REMOVED_FEATURES`, so there isn't really a point in changing anything there now.

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