c++: empty base constexpr -fno-elide-ctors [PR105245]

c++: empty base constexpr -fno-elide-ctors [PR105245]

The patch for 100111 extended our handling of empty base elision to the case
where the derived class has no other fields, but we still need to make sure
that there's some initializer for the derived object.

	PR c++/105245
	PR c++/100111

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_store_expression): Build a CONSTRUCTOR
	as needed in empty base handling.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/constexpr-empty2.C: Add -fno-elide-constructors.