c++: Fix crash in gimplifier with paren init of aggregates [PR94155]

c++: Fix crash in gimplifier with paren init of aggregates [PR94155]

Here we crash in the gimplifier because gimplify_init_ctor_eval doesn't
expect null indexes for a constructor:

      /* ??? Here's to hoping the front end fills in all of the indices,
         so we don't have to figure out what's missing ourselves.  */
      gcc_assert (purpose);

The indexes weren't filled because we never called reshape_init: for
a constructor that represents parenthesized initialization of an
aggregate we don't allow brace elision or designated initializers.

	PR c++/94155 - crash in gimplifier with paren init of aggregates.
	* init.c (build_vec_init): Fill in indexes.

	* g++.dg/cpp2a/paren-init22.C: New test.