Implement P1274R4 Reviewed By: Quuxplusone, Mordante, #libc Spies: jloser, lebedev.ri, mgorny, libcxx-commits, arichardson Differential Revision: https://reviews.llvm.org/D114074
Thanks to Arthur O'Dwyer for fixing up some of the tests. Differential Revision: https://reviews.llvm.org/D75960