c++: Avoid redundant copy in {} init [PR98642]

Here, initializing from { } implies a call to the default constructor for
base.  We were then seeing that we're initializing a base subobject, so we
tried to copy the result of that call.  This is clearly wrong; we should
initialize the base directly from its default constructor.

This patch does a lot of refactoring of unsafe_copy_elision_p and adds
make_safe_copy_elision that will also try to do the base constructor
rewriting from the last patch.

gcc/cp/ChangeLog:

	PR c++/98642
	* call.c (unsafe_return_slot_p): Return int.
	(init_by_return_slot_p): Split out from...
	(unsafe_copy_elision_p): ...here.
	(unsafe_copy_elision_p_opt): New name for old meaning.
	(build_over_call): Adjust.
	(make_safe_copy_elision): New.
	* typeck2.c (split_nonconstant_init_1): Elide copy from safe
	list-initialization.
	* cp-tree.h: Adjust.

gcc/testsuite/ChangeLog:

	PR c++/98642
	* g++.dg/cpp1z/elide5.C: New test.