[libc++][format] Implements formattable concept.

This concept is introduced in P2286, but was implemented in libc++
before. This implementation was used in the library internally. This
implementation lacked the resolution of LWG3636. The original formatter
had a non-const member function that wasn't trivial to make a const
member. The recent parser improvements made this member a const member
in preparation of LWG3636.

Note LWG3636 isn't voted in. Its status is Ready. P2286's concept has
been written as-if LWG3636 is accepted and refers to that LWG issue.

Updates some tests make format a const member function and removes a
tests that's mainly a duplicate of the formattable concept test.

Implements
- LWG3636 formatter<T>::format should be const-qualified

Implements parts of
- P2286R8 Formatting Ranges

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D134110