PR libstdc++/79980
	* include/bits/locale_conv.h (__do_str_codecvt): Set __count on
	error path.
	* src/c++11/codecvt.cc (operator&=, operator|=, operator~): Overloads
	for manipulating codecvt_mode values.
	(read_utf16_bom): Compare input to BOM constants instead of integral
	constants that depend on endianness.  Take mode parameter by
	reference and adjust it, to distinguish between no BOM present and
	UTF-16BE BOM present.
	(ucs4_in, ucs2_span, ucs4_span): Adjust calls to read_utf16_bom.
	(surrogates): New enumeration type.
	(utf16_in, utf16_out): Add surrogates parameter to choose between
	UTF-16 and UCS2 behaviour.
	(utf16_span, ucs2_span): Use std::min not std::max.
	(ucs2_out): Use std::min not std::max.  Disallow surrogate pairs.
	(ucs2_in): Likewise. Adjust calls to read_utf16_bom.
	* testsuite/22_locale/codecvt/codecvt_utf16/79980.cc: New test.
	* testsuite/22_locale/codecvt/codecvt_utf8/79980.cc: New test.

From-SVN: r246200