libstdc++: Fix Unicode codecvt and add tests [PR86419]

libstdc++: Fix Unicode codecvt and add tests [PR86419]

Fixes the conversion from UTF-8 to UTF-16 to properly return partial
instead ok.
Fixes the conversion from UTF-16 to UTF-8 to properly return partial
instead ok.
Fixes the conversion from UTF-8 to UCS-2 to properly return partial
instead error.
Fixes the conversion from UTF-8 to UCS-2 to treat 4-byte UTF-8 sequences
as error just by seeing the leading byte.
Fixes UTF-8 decoding for all codecvts so they detect error at the end of
the input range when the last code point is also incomplete.

libstdc++-v3/ChangeLog:

	PR libstdc++/86419
	* src/c++11/codecvt.cc (read_utf8_code_point): Correctly detect
	errors in incomplete multibyte sequences.
	(utf16_in): Remove surrogates parameter. Fix conditions for
	returning partial.
	(utf16_out): Fix condition for returning partial.
	(ucs2_in): Do not pass surrogates argument to utf16_in.
	* testsuite/22_locale/codecvt/codecvt_unicode.cc: New test.
	* testsuite/22_locale/codecvt/codecvt_unicode.h: New header for
	tests.
	* testsuite/22_locale/codecvt/codecvt_unicode_wchar_t.cc: New
	test.

(cherry picked from commit 02dab998665dda0f6df31740e8897c42de3d740f)