c++: initialized array of vla [PR58646]

We went into build_vec_init because we're dealing with a VLA, but then
build_vec_init thought it was safe to just build an INIT_EXPR because the
outer dimension is constant.  Nope.

	PR c++/58646

gcc/cp/ChangeLog:

	* init.cc (build_vec_init): Check for vla element type.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/vla24.C: New test.