Enable std::pointer_traits speciailzation for raw_ref.

This similar to one of my previous patches for raw_ptr [1]. The
code is not really libc++ specific, it works the same for libstdc++.
The are no failing gtests due to this, but there are no good reasons
to keep the behavior different between libc++ and libstdc++. Drop the
_LIBCPP_VERSION ifdefs.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/4555577

Bug: 957519
Change-Id: I1a22674709498bab7d17e18f6917e77e544c8846
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4558428


Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Commit-Queue: Piotr Tworek <piotr.tworek@xperi.com>
Cr-Commit-Position: refs/heads/main@{#1148494}