[libc++][format] Adds formattable-with concept.

[libc++][format] Adds formattable-with concept.

This change has a few additional effects:
- Abstract classes are now formattable.
- Volatile objects are no longer formattable.

Implements
- LWG3631 basic_format_arg(T&&) should use remove_cvref_t<T> throughout
- LWG3925 Concept formattable's definition is incorrect

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D152092