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

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

This implements the last required formatter specialization.

Completes:
- LWG 3251 Are std::format alignment specifiers applied to string arguments?
- LWG 3340 Formatting functions should throw on argument/format string mismatch in §[format.functions]
- LWG 3540 §[format.arg] There should be no const in basic_format_arg(const T* p)

Implements parts of:
- P0645 Text Formatting

Depends on D114001

Reviewed By: ldionne, vitaut, #libc

Differential Revision: https://reviews.llvm.org/D115988