[libc++][NFC] Refactor the __enable_ifs in <string>

This changes all `__enable_if`s inside `<string>` to a common pattern. Specifically, it's always inside the `template <>` and uses the `, int> = 0` style.

Reviewed By: ldionne, #libc

Spies: mikhail.ramalho, EricWF, libcxx-commits

Differential Revision: https://reviews.llvm.org/D144568