[clang][dataflow] Only skip ExprWithCleanups when visiting terminators

`IgnoreParenImpCasts` will remove implicit casts to bool
(e.g. `PointerToBoolean`), such that the resulting expression may not
be of the `bool` type. The `cast_or_null<BoolValue>` in
`extendFlowCondition` will then trigger an assert, as the pointer
expression will not have a `BoolValue`.

Instead, we only skip `ExprWithCleanups` and `ParenExpr` nodes, as the
CFG does not emit them.

Differential Revision: https://reviews.llvm.org/D124807