[libc++] Improves clang-format settings.

Add a new test based .clang-format file which inherits from the generic
one. This moves some test specific formatting rules to the test
directory.

The main benefit is that headers are sorted, which makes it more likely
to catch these errors before creating a review instead of spotting the
error in the CI clang-tidy step.

Reviewed By: ldionne, philnik, #libc

Differential Revision: https://reviews.llvm.org/D144755