PR libstdc++/64797
	* include/bits/locale_conv.h (wstring_convert::_M_conv): Handle
	incomplete multibyte sequences correctly.
	* include/std/codecvt (codecvt_utf8, codecvt_utf16,
	codecvt_utf8_utf16): Limit _Maxcode to maximum Unicode code point.
	* src/c++11/codecvt.cc (invalid_mb_sequence, incomplete_mb_character):
	Define constants.
	(is_high_surrogate, is_low_surrogate, surrogate_pair_to_code_point):
	Define convenience functions.
	(read_utf8_code_point): Return relevant constant to distinguish
	incomplete characters from invalid sequences.
	(read_utf16_code_point): Likewise. Check for invalid sequences.
	(ucs4_in, utf16_in): Use incomplete_mb_character constant.
	(utf16_out): Check for invalid sequences.
	(utf16_span): Fix condition.
	(ucs2_out): Use is_high_surrogate.
	(ucs2_in): Use incomplete_mb_character constant and fix condition.
	* testsuite/22_locale/codecvt/char16_t.cc: Fix whitespace.
	* testsuite/22_locale/conversions/buffer/1.cc: New.
	* testsuite/22_locale/conversions/string/2.cc: Use char16_t and
	char32_t instead of wchar_t.
	* testsuite/22_locale/conversions/string/3.cc: New.

From-SVN: r221189