Fix encoding() and max_length() values for codecvt facets

Fix encoding() and max_length() values for codecvt facets

	* src/c++11/codecvt.cc (codecvt<char16_t, char, mbstate_t>)
	(codecvt<char32_t, char, mbstate_t>, __codecvt_utf8_base<char16_t>)
	(__codecvt_utf8_base<char32_t>, __codecvt_utf8_base<wchar_t>)
	(__codecvt_utf16_base<char16_t>, __codecvt_utf16_base<char32_t>)
	(__codecvt_utf16_base<wchar_t>, __codecvt_utf8_utf16_base<char16_t>)
	(__codecvt_utf8_utf16_base<char32_t>)
	(__codecvt_utf8_utf16_base<wchar_t>): Fix do_encoding() and
	do_max_length() return values.
	* testsuite/22_locale/codecvt/codecvt_utf16/members.cc: New test.
	* testsuite/22_locale/codecvt/codecvt_utf8/members.cc: New test.
	* testsuite/22_locale/codecvt/codecvt_utf8_utf16/members.cc: New test.

From-SVN: r246201