Make sure TraverseInitListExpr visits InitListExpr exactly twice

... once each for the syntactic and semantic form. Without this fix, behavior
of the newly added tests would have been

InitListExprIsPreOrderVisitedTwice:
 syntactic: 1
 semantic: 2

InitListExprIsPostOrderVisitedTwice:
 syntactic: 0
 semantic: 1

InitListExprIsPreOrderNoQueueVisitedTwice:
 syntactic: 1
 semantic: 2

InitListExprIsPostOrderNoQueueVisitedTwice:
 syntactic: 0
 semantic: 2

llvm-svn: 306374