c++: constexpr non-trivial aggregate init [PR105191]

c++: constexpr non-trivial aggregate init [PR105191]

My patch for PR92385 made us use VEC_INIT_EXPR for aggregate initialization
of an array where some elements are not explicitly initialized.  Constexpr
handling of that was treating initialization from {} as equivalent to
value-initialization, which is problematic for classes with default member
initializers that make the default constructor non-trivial; in older
standard modes, not initializing all members makes a constructor
non-constexpr, but aggregate initialization is fine.

	PR c++/105191
	PR c++/92385

gcc/cp/ChangeLog:

	* tree.cc (build_vec_init_elt): Do {}-init for aggregates.
	* constexpr.cc (cxx_eval_vec_init): Only treat {} as value-init
	for non-aggregate types.
	(build_vec_init_expr): Also check constancy of explicit
	initializer elements.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-array28.C: New test.