libstdc++: Replace non-type-dependent uses of wchar_t in <format> and <chrono>

This is one more piece of the rework to make wchar_t support in
std::format depend on _GLIBCXX_USE_WCHAR_T.

In <format> the __to_wstring_numeric function is called with arguments
that aren't type-dependent, so a declaration needs to be available, or
the calls need to be guarded by _GLIBCXX_USE_WCHAR_T.

In <chrono> there is a similarly non-type-dependent call to std::format
with a wchar_t format string, which is ill-formed when the wchar_t
overloads of std::format are not declared. Use _GLIBCXX_WIDEN to make it
type-dependent.

libstdc++-v3/ChangeLog:

	* include/bits/chrono_io.h (operator<<): Make uses of wide
	strings with streams and std::format type-dependent on _CharT.
	* include/std/format [!_GLIBCXX_USE_WCHAR_T] Do not use
	__to_wstring_numeric.