[libc++] Implement P2438R2 (std::string::substr() &&)

This doesn't affect our ABI because `std::string::substr()` isn't in the dylib and the mangling of `substr() const` and `substr() const&` are different.

Reviewed By: ldionne, Mordante, var-const, avogelsgesang, #libc

Spies: arphaman, huixie90, libcxx-commits

Differential Revision: https://reviews.llvm.org/D131668