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

[philnik]

Summary

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

[EricWF]

This increases code duplication substantially.

Please say why this refactoring is an improvement.

[EricWF]

@philnik Every single change needs a description. The description must include the motivation for making the change. What does in improve? What code does it break/fix? etc.

This is important for review, and its even more important down the road when we're trying to figure out why a particular change was made all that time ago.

[ldionne]

I am fine with this since:

1. It increases consistency with the way we use enable_if elsewhere, and
2. the duplication is going away once we implement those member functions in the class, which we should do anyway.

libcxx/include/string

2053

There's a bit of duplication here, which is kind of annoying. However, refactoring this short function to be defined in-class will remove that duplication, and also remove some boilerplate which is not really useful.

[philnik]

Rebased