middle-end/106909 - CTRL altering flag after folding

The following makes sure to clear the CTRL altering flag when
folding emits a __builitin_unreachable in place of a virtual call
which now might become a trap.

	PR middle-end/106909
	* gimple-fold.cc (gimple_fold_call): Clear the ctrl-altering
	flag of a unreachable call.