c++: ICE casting class to vector [PR100517]

My recent change to reject calling rvalue() with an argument of class type
crashes on this testcase, where we use rvalue() on what we expect to be an
argument of integer or vector type.  Fixed by checking first.

gcc/cp/ChangeLog:

	PR c++/100517
	* typeck.c (build_reinterpret_cast_1): Check intype on
	cast to vector.

gcc/testsuite/ChangeLog:

	PR c++/100517
	* g++.dg/ext/vector41.C: New test.