libstdc++: Fix handling of surrogate CP in codecvt [PR108976]

libstdc++: Fix handling of surrogate CP in codecvt [PR108976]

This patch fixes the handling of surrogate code points in all standard
facets for transcoding Unicode that are based on std::codecvt. Surrogate
code points should always be treated as error. On the other hand
surrogate code units can only appear in UTF-16 and only when they come
in a proper pair.

Additionally, it fixes a bug in std::codecvt_utf16::in() when odd number
of bytes were given in the range [from, from_end), error was returned
always. The last byte in such range does not form a full UTF-16 code
unit and we can not make any decisions for error, instead partial should
be returned.

The testsuite for testing these facets was updated in the following
order:

1. All functions that test codecvts that work with UTF-8 were refactored
   and made more generic so they accept codecvt that works with the char
   type char8_t.
2. The same functions were updated with new test cases for transcoding
   errors and now additionally test for surrogates, overlong UTF-8
   sequences, code points out of the Unicode range, and more tests for
   missing leading and trailing code units.
3. New tests were added to test codecvt_utf16 in both of its variants,
   UTF-16 <-> UTF-32/UCS-4 and UTF-16 <-> UCS-2.

libstdc++-v3/ChangeLog:

	PR libstdc++/108976
	* src/c++11/codecvt.cc (read_utf8_code_point): Fix handing of
	surrogates in UTF-8.
	(ucs4_out): Fix handling of surrogates in UCS-4 -> UTF-8.
	(ucs4_in): Fix handling of range with odd number of bytes.
	(ucs4_out): Fix handling of surrogates in UCS-4 -> UTF-16.
	(ucs2_out): Fix handling of surrogates in UCS-2 -> UTF-16.
	(ucs2_in): Fix handling of range with odd number of bytes.
	(__codecvt_utf16_base<char16_t>::do_in): Likewise.
	(__codecvt_utf16_base<char32_t>::do_in): Likewise.
	(__codecvt_utf16_base<wchar_t>::do_in): Likewise.
	* testsuite/22_locale/codecvt/codecvt_unicode.cc: Renames, add
	tests for codecvt_utf16<char16_t> and codecvt_utf16<char32_t>.
	* testsuite/22_locale/codecvt/codecvt_unicode.h: Refactor UTF-8
	testing functions for char8_t, add more test cases for errors,
	add testing functions for codecvt_utf16.
	* testsuite/22_locale/codecvt/codecvt_unicode_wchar_t.cc:
	Renames, add tests for codecvt_utf16<whchar_t>.
	* testsuite/22_locale/codecvt/codecvt_utf16/79980.cc (test06):
	Fix test.
	* testsuite/22_locale/codecvt/codecvt_unicode_char8_t.cc: New
	test.