c++: Poor diagnostic for dynamic_cast in constexpr context [PR93414]

c++: Poor diagnostic for dynamic_cast in constexpr context [PR93414]

I neglected to add a proper diagnostic for the reference dynamic_cast
case when the operand of a dynamic_cast doesn't refer to a public base
of Derived, resulting in suboptimal error message

   error: call to non-'constexpr' function 'void* __cxa_bad_cast()'

2020-01-25  Marek Polacek  <polacek@redhat.com>

	PR c++/93414 - poor diagnostic for dynamic_cast in constexpr context.
	* constexpr.c (cxx_eval_dynamic_cast_fn): Add a reference
	dynamic_cast diagnostic.

	* g++.dg/cpp2a/constexpr-dynamic18.C: New test.