re PR c++/79650 (ICE on invalid c++ code with label arithmetic in convert_nontype_argument (pt.c:6515))

	PR c++/79650
	* pt.c (convert_nontype_argument): Diagnose
	reduced_constant_expression_p expressions that aren't INTEGER_CST.

	* g++.dg/template/pr79650.C: New test.

From-SVN: r255623