c++: constexpr empty base redux [PR105622]

c++: constexpr empty base redux [PR105622]

Here calling the constructor for s.__size_ had ctx->ctor for s itself
because cxx_eval_store_expression doesn't create a ctor for the empty field.
Then cxx_eval_call_expression returned the s initializer, and my empty base
overhaul in r13-160 got confused because the type of init is not an empty
class.  But that's OK, we should be checking the type of the original LHS
instead.  We also want to use initialized_type in the condition, in case
init is an AGGR_INIT_EXPR.

I spent quite a while working on more complex solutions before coming back
to this simple one.

	PR c++/105622

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_store_expression): Adjust assert.
	Use initialized_type.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/no_unique_address14.C: New test.