c++: Fix list-init of array of no-copy type [PR63707]

build_vec_init_elt models initialization from some arbitrary object of the
type, i.e. copy, but in the case of list-initialization we don't do a copy
from the elements, we initialize them directly.

gcc/cp/ChangeLog:

	PR c++/63707
	* tree.c (build_vec_init_expr): Don't call build_vec_init_elt
	if we got a CONSTRUCTOR.

gcc/testsuite/ChangeLog:

	PR c++/63707
	* g++.dg/cpp0x/initlist-array13.C: New test.