c-family: avoid compile-time-hog in c_genericize [PR108880]

c-family: avoid compile-time-hog in c_genericize [PR108880]

This fixes a compile-time hog with UBSan.  This only happened in cc1 but
not cc1plus.  The problem is ultimately that c_genericize_control_stmt/
STATEMENT_LIST -> walk_tree_1 doesn't use a hash_set to remember visited
nodes, so it kept on recursing for a long time.  We should be able to
use the pset that c_genericize created.  We just need to use walk_tree
instead of walk_tree_w_d so that the pset is explicit.

	PR c/108880

gcc/c-family/ChangeLog:

	* c-gimplify.cc (c_genericize_control_stmt) <case STATEMENT_LIST>: Pass
	pset to walk_tree_1.
	(c_genericize): Call walk_tree with an explicit pset.

gcc/testsuite/ChangeLog:

	* c-c++-common/ubsan/pr108880.c: New test.