[libc++][NFC] Refactor enable_ifs in defaulted arguments to defaulted template arguments

This brings most of the enable_ifs in libc++ to the same style. It also has the nice side-effect of reducing the size of names of these symbols, since the arguments don't get mangled anymore.

Reviewed By: #libc, Mordante

Spies: Mordante, libcxx-commits

Differential Revision: https://reviews.llvm.org/D157748