PR libstdc++/80624
	* doc/xml/manual/status_cxx2011.xml: Document to_int_type behaviour.
	* include/bits/char_traits.h (char_traits<char16_t>::to_int_type):
	Transform eof value to U+FFFD.
	* testsuite/21_strings/char_traits/requirements/char16_t/eof.cc: New.
	* testsuite/27_io/basic_streambuf/sgetc/char16_t/80624.cc: New.
	* testsuite/27_io/basic_streambuf/sputc/char16_t/80624.cc: New.

From-SVN: r248843