[libc++] Update clang-format to C++20.

We now use clang-format-13 which has the option SpacesInAngles. This
allows us to switch the default language version to C++20, which should
avoid breaking code when formatting due to the adding of whitespace.
For example `u8"foo"` no longer is formatted as `u8 "foo"`.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D112728