c++: constexpr evaluation of abi::__dynamic_cast [PR120620]

c++: constexpr evaluation of abi::__dynamic_cast [PR120620]

r13-3299 changed our internal declaration of __dynamic_cast to reside
inside the abi/__cxxabiv1:: namespace instead of the global namespace,
matching the real declaration.  This inadvertently made us now attempt
constexpr evaluation of user-written calls to abi::__dynamic_cast since
cxx_dynamic_cast_fn_p now also returns true for them, but we're not
prepared to handle arbitrary calls to __dynamic_cast, and therefore ICE.

This patch restores cxx_dynamic_cast_fn_p to return true only for
synthesized calls to __dynamic_cast, which can be distinguished by
DECL_ARTIFICIAL, since apparently the synthesized declaration of
__dynamic_cast doesn't get merged with the actual declaration.

	PR c++/120620

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_dynamic_cast_fn_p): Return true only
	for synthesized __dynamic_cast.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/constexpr-dynamic19.C: New test.
	* g++.dg/cpp2a/constexpr-dynamic1a.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>