c++: ICE with reinterpret_cast and switch [PR113545]

Jason, this is the patch you proposed for PR113545.  It looks very safe
so I'm posting it here so that it's not forgotten.

	PR c++/113545

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_switch_expr): If the condition doesn't reduce
	to an INTEGER_CST, consider it non-constant.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/constexpr-reinterpret3.C: Remove dg-ice.