[libcxx] Make sure reference_wrapper works with incomplete types

Summary: Completes P0357R3, which was merged into the C++20 Working Draft in San Diego.

Reviewers: EricWF, mclow.lists

Subscribers: christof, jkorous, dexonsmith, libcxx-commits

Differential Revision: https://reviews.llvm.org/D54722

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@357423 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed