[libc++] Find a clang-format everybody is happy with

We decided that we want to use clang-format for libc++ but we haven't decided yet how the code should be formatted. We should probably discuss things on discord. This PR is mostly to show how the clang-format would look like and to commit to one once we decided on it. I'll remove the `<string>` diff when I commit this PR.

Reviewed By: ldionne

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

Differential Revision: https://reviews.llvm.org/D124789