[libc++][NFC] Refactor enable_ifs in vector

Using the `enable_if_t<..., int> = 0` style has the benefit that it works in all cases and makes function declarations easier to read because the function arguments and return type and SFINAE are separated. Unifying the style also makes it easier for people not super familiar with SFINAE to make sense of the code.

Reviewed By: Mordante, var-const, #libc, huixie90

Spies: huixie90, libcxx-commits

Differential Revision: https://reviews.llvm.org/D131868