libstdc++: Add missing constexpr specifiers in <format>

A couple of virtual functions in the libstdc++ format header are marked
constexpr in the base class, but not in the derived class. This was
causing build failures when trying to compile latest gcc libstdc++ with
clang 16 using c++20. Adding the constexpr specifier resolves the issue.

libstdc++-v3/ChangeLog:

	* include/std/format (_Formatting_scanner::_M_on_chars): Add
	missing constexpr specifier.
	(_Formatting_scanner::_M_format_arg): Likewise.