Introduce the `ScopeCloser` and `GotoTarget` annotations in the IR, and
the relative necessary machinery, needed to handle scope closer and goto
edges for the new backend.
A specialization of the `llvm::GraphTraits`, called `ScopeGraph`, that
is able to handle both the above mentioned annotations is provided.
For the `llvm::GraphTraits` implementation, we introduce the
`GeneratorIterator` class, which uses a coroutine to store the status of
the iteration.
A debug logger pass is added, so that we are able to test the
functionality with `FileCheck`.
Andrea Gussoni