c++: build initializer_list<string> in a loop [PR105838]

The previous patch avoided building an initializer_list<string> at all when
building a vector<string>, but in situations where that isn't possible, we
could still build the initializer_list with a loop over a constant array.

This is represented using a VEC_INIT_EXPR, which required adjusting a couple
of places that expected the initializer array to have the same type as the
target array and fixing build_vec_init not to undo our efforts.

	PR c++/105838

gcc/cp/ChangeLog:

	* call.cc (convert_like_internal) [ck_list]: Use
	maybe_init_list_as_array.
	* constexpr.cc (cxx_eval_vec_init_1): Init might have
	a different type.
	* tree.cc (build_vec_init_elt): Likewise.
	* init.cc (build_vec_init): Handle from_array from a
	TARGET_EXPR.  Retain TARGET_EXPR of a different type.

gcc/testsuite/ChangeLog:

	* g++.dg/tree-ssa/initlist-opt2.C: New test.