[libc++] Add format check to CI

Note: contrary to what I said previously, I didn't change .clang-format nor utils/generate_feature_test_macro_components.py script.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D92229

GitOrigin-RevId: 1361c5e7d703607e4ae8bc48201ead4cdb5acdb9
2 files changed