[libc++][NFC] Rename iterator category checks to make it obvious that they...

[libc++][NFC] Rename iterator category checks to make it obvious that they check //only// the iterator category

We plan to add concepts for checking that iterators actually provide what they claim to. This is to avoid people thinking that these type traits actually check the iterator requirements in more detail.

Reviewed By: ldionne, #libc

Spies: Mordante, libcxx-commits, wenlei

Differential Revision: https://reviews.llvm.org/D150801