PR c++/91370 - Implement P1041R4 and P1139R2 - Stronger Unicode reqs

PR c++/91370 - Implement P1041R4 and P1139R2 - Stronger Unicode reqs

	PR c++/91370 - Implement P1041R4 and P1139R2 - Stronger Unicode reqs
	* charset.c (narrow_str_to_charconst): Add TYPE argument.  For
	CPP_UTF8CHAR diagnose whenever number of chars is > 1, using
	CPP_DL_ERROR instead of CPP_DL_WARNING.
	(wide_str_to_charconst): For CPP_CHAR16 or CPP_CHAR32, use
	CPP_DL_ERROR instead of CPP_DL_WARNING when multiple char16_t
	or char32_t chars are needed.
	(cpp_interpret_charconst): Adjust narrow_str_to_charconst caller.

	* g++.dg/cpp1z/utf8-neg.C: Expect errors rather than -Wmultichar
	warnings.
	* g++.dg/ext/utf16-4.C: Expect errors rather than warnings.
	* g++.dg/ext/utf32-4.C: Likewise.
	* g++.dg/cpp2a/ucn2.C: New test.

From-SVN: r277929