re PR c++/70847 (exponential time in cp_fold for chained virtual function calls)

re PR c++/70847 (exponential time in cp_fold for chained virtual function calls)

	PR c++/70847
	PR c++/71330
	PR c++/71393
	* cp-gimplify.c (cp_fold_r): Set *walk_subtrees = 0 and return NULL
	right after cp_fold call if cp_fold has returned the same stmt
	already in some earlier cp_fold_r call.
	(cp_fold_function): Add pset automatic variable, pass its address
	to cp_walk_tree.

	* g++.dg/opt/pr70847.C: New test.
	* g++.dg/ubsan/pr70847.C: New test.
	* g++.dg/ubsan/pr71393.C: New test.

Co-Authored-By: Patrick Palka <ppalka@gcc.gnu.org>
From-SVN: r237151