[libc++] Enforce formatting for already formatted and new files and ignore the formatting of tests

It is quite confusing to newcomers that the formatting check gets mostly ignored. To fix that, enforce the formatting for new file and already formatted files, but ignore it for any files that aren't formatted already. We ignore the tests for now, since almost no test is formatted currently, and they are changed almost never.

Reviewed By: ldionne, Mordante, #libc

Spies: arichardson, fedor.sergeev, phosek, sstefan1, libcxx-commits, abrachet

Differential Revision: https://reviews.llvm.org/D142180