`visitTopLevelCodeDecl` ignored the `Stmt` visit returning a nullptr.
This caused the `walkToDeclPost` to run for the `TopLevelCodeDecl` and
thus an imbalance in the `RangeResolver` pre and posts (since none of
the children would have their `walkTo*Post` called).
This was originally incorrectly fixed while assuming that the
`walkTo*Post` are called regardless of whether the children were visited
or not. Those changes have been reverted - fixing an imbalance in
`ExtDecls` in `SemaAnnotator`.
Added a test case for the `ExtDecls` imbalance which can occur while an
extension is being added.
Resolves rdar://74820040
