c++: constexpr virtual and vbase thunk

c++: constexpr virtual and vbase thunk

C++20 allows virtual functions to be constexpr.  I don't think that calling
through a pointer to a vbase subobject is supposed to work in a constant
expression, since an object with virtual bases can't be constant, but the
call shouldn't ICE.

gcc/cp/ChangeLog:

	* constexpr.c (cxx_eval_thunk_call): Error instead of ICE
	on vbase thunk to constexpr function.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/constexpr-virtual20.C: New test.