[libc++][format] Improves fill character.

[libc++][format] Improves fill character.

The main change is to allow a UCS scalar value as fill character.
Especially for char based formatting this increase the number of valid
characters. Originally this was to be expected ABI breaking, however the
current change does not seem to break the ABI.

Implements
- P2572 std::format() fill character allowances

Depends on D144499

Reviewed By: ldionne, tahonermann, #libc

Differential Revision: https://reviews.llvm.org/D144742