[libc++][format] Adds formatter handle.

This implements the handler according to P0645. P2418 changes the wording
in the Standard. That isn't implemented and requires changes in more
places. LWG3631 applies modifications to P2418, but is currently
unresolved.

Implements parts of:
* P0645 Text Formatting

Depends on D115989

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D115991