c++: Fix copy elision for base initialization

While working on PR98642 I noticed that in this testcase we were eliding the
copy, calling the complete default constructor to initialize the B base
subobject, and therefore wrongly initializing the non-existent A subobject
of B.  The test doesn't care whether the copy is elided or not, but checks
that we are actually calling a base constructor for B.

The patch preserves the elision, but changes the initializer to call the
base constructor instead of the complete constructor.

gcc/cp/ChangeLog:

	* call.c (base_ctor_for, make_base_init_ok): New.
	(build_over_call): Use make_base_init_ok.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/elide4.C: New test.