c++: no_unique_address and constexpr [PR112439]

c++: no_unique_address and constexpr [PR112439]

Here, because we don't build a CONSTRUCTOR for an empty base, we were
wrongly marking the Foo CONSTRUCTOR as complete after initializing the Empty
member.  Fixed by checking empty_base here as well.

	PR c++/112439

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_store_expression): Check empty_base
	before marking a CONSTRUCTOR readonly.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/no_unique_address15.C: New test.