[flang] Fix bugs detecting impure calls

Change Traverse to visit the actual arguments of structure constructors.

Change FindImpureCallHelper to visit the actual arguments of a call to a
pure procedure in case one of them makes a call to an impure function.

Original-commit: flang-compiler/f18@81a5488ee62b4324d002c348464712c930095a32
Reviewed-on: https://github.com/flang-compiler/f18/pull/951