libstdc++: Fix handling of field width for wide strings and characters [PR119593]

libstdc++: Fix handling of field width for wide strings and characters [PR119593]

This patch corrects handling of UTF-32LE and UTF32-BE in
__unicode::__literal_encoding_is_unicode<_CharT>, so they are
recognized as unicode and functions produces correct result for wchar_t.

Use `__unicode::__field_width` to compute the estimated witdh
of the charcter for unicode wide encoding.

	PR libstdc++/119593

libstdc++-v3/ChangeLog:

	* include/bits/unicode.h
	(__unicode::__literal_encoding_is_unicode<_CharT>):
	Corrected handing for UTF-16 and UTF-32 with "LE" or "BE" suffix.
	* include/std/format (__formatter_str::_S_character_width):
	Define.
	(__formatter_str::_S_character_width): Updated passed char
	length.
	* testsuite/std/format/functions/format.cc: Test for wchar_t.

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