[libc++] Implement P1072R10 (std::basic_string::resize_and_overwrite)

Reviewed By: Quuxplusone, #libc, Mordante

Spies: mzeren-vmw, ckennelly, arichardson, ldionne, Mordante, libcxx-commits, Quuxplusone

Differential Revision: https://reviews.llvm.org/D113013