c++: Fix up VEC_INIT_EXPR gimplification after r12-7069

c++: Fix up VEC_INIT_EXPR gimplification after r12-7069

During patch backporting, I've noticed that while most cp_walk_tree calls
with cp_fold_r callback callers were changed from &pset to cp_fold_data
&data, the VEC_INIT_EXPR gimplifications has not, so it still passes just
address of a hash_set<tree> and so if during the folding we ever touch
data->flags, we use uninitialized data there.

The following patch changes it to do the same thing as cp_fold_function
because the VEC_INIT_EXPR gimplifications will happen on function bodies
only.

2023-05-03  Jakub Jelinek  <jakub@redhat.com>

	* cp-gimplify.cc (cp_fold_data): Move definition earlier.
	(cp_gimplify_expr): Pass address of ff_genericize | ff_mce_false
	constructed data rather than &pset to cp_walk_tree with cp_fold_r.

(cherry picked from commit 8d193b12d6f07ae0196db8296a49c881c1638c01)