[libc++][NFC] clang-format <__config>

It's not perfect, but it's a lot better than the status quo.

Reviewed By: ldionne, #libc

Spies: EricWF, aheejin, libcxx-commits, dschuff, krytarowski, fedor.sergeev, mstorsjo, phosek, abrachet

Differential Revision: https://reviews.llvm.org/D127644