c++: [[no_unique_address]] in empty base [PR98463]

c++: [[no_unique_address]] in empty base [PR98463]

In this testcase, cxx_eval_store_expression got confused trying to build up
CONSTRUCTORs for initializing a subobject because the subobject is a member
of an empty base.  In C++14 mode and below we don't build FIELD_DECLs for
empty bases, so the CONSTRUCTOR skipped the empty base, and treated the
member as a member of the derived class, which breaks.

Fixed by recognizing this situation and giving up on trying to build a
CONSTRUCTOR for the inner target at that point; since it doesn't have any
data, we don't need to actually store anything.

gcc/cp/ChangeLog:

	PR c++/98463
	* constexpr.c (get_or_insert_ctor_field): Add check.
	(cxx_eval_store_expression): Handle discontinuity of refs.

gcc/testsuite/ChangeLog:

	PR c++/98463
	* g++.dg/cpp2a/no_unique_address8.C: New test.