[libc++] Make chars_format a bitmask type.

Some of Microsoft's unit tests in D70631 fail because libc++'s
implementation of std::chars_format isn't a proper bitmask type. Adding
the required functions to make std::chars_format a proper bitmask type.

Implements parts of P0067: Elementary string conversions

Differential Revision: https://reviews.llvm.org/D97115