c++: unifying REAL_CSTs [PR110809]

This teaches unify how to compare two REAL_CSTs.

	PR c++/110809

gcc/cp/ChangeLog:

	* pt.cc (unify) <case INTEGER_CST>: Generalize to handle
	REAL_CST as well.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/nontype-float3.C: New test.