ipa/120146 - deal with vanished varpool nodes in IPA PTA

I don't understand why they vanish when still refered to, but
lets deal with that in a conservative way.

	PR ipa/120146
	* tree-ssa-structalias.cc (create_variable_info_for): If
	the symtab cannot tell us whether all refs to a variable
	are explicit assume they are not.

	* g++.dg/ipa/pr120146.C: New testcase.

(cherry picked from commit b38e3a7196d25bc8bcb1fe55da7663745cea9470)