libstdc++: Fix width computation for the chrono formatting [PR120114]

libstdc++: Fix width computation for the chrono formatting [PR120114]

Use `__unicode::_field_width` to compute the field width of the output when writting
the formatted output for std::chrono::types. This applies both to characters copied
from format string, and one produced by localized formatting.

We also use _Str_sink::view() instead of get(), which avoids copying the content of
the buffer to std::string in case of small output.

	PR libstdc++/120114

libstdc++-v3/ChangeLog:

	* include/bits/chrono_io.h (__formatter_chrono::_M_format): Use __field_width.
	* testsuite/std/time/format/pr120114.cc: New test.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>