[libcxx] Implement P0318: unwrap_ref_decay and unwrap_reference

Summary:
This was voted into C++20 in San Diego. Note that there was a revision
D0318R2 which did include unwrap_reference_t, but we mistakingly voted
P0318R1 into the C++20 Working Draft (which does not include
unwrap_reference_t). This patch implements D0318R2, which is what
we'll end up with in the Working Draft once this mistake has been
fixed.

Reviewers: EricWF, mclow.lists

Subscribers: christof, dexonsmith, libcxx-commits

Differential Revision: https://reviews.llvm.org/D54485

llvm-svn: 348138