[libc++] Applies P0602R4 retro-actively.

While testing a test failure of C++17 with Clang ToT it was noticed the
paper
  P0602R4 variant and optional should propagate copy/move triviality
was not applied as a DR in libc++.

This was discovered while investigating the issue "caused by" D131479.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D133326