Many classes have been declared with `extends @cfgnode` because they
should be implemented internally as a control-flow node but should not
expose the member predicates of `ControlFlowNode` to their users. After
the transition in a1e44041e it became mandatory to convert explicitly
between the `Element`-derived `ControlFlowNode` and the raw dbtype
`@cfgnode`, and that commit inserted numerous such conversions as a
result of having all those classes that did not derive from `Element` in
the standard library.
It was also confusing and error-prone that the libraries implementing
`ControlFlowNode` referred to `ControlFlowNode`. This seemingly cyclic
reference worked out because the libraries did not call the predicates
on `ControlFlowNode` whose implementation they were part of.
Both these problems are now solved by adding a new class
`ControlFlowNodeBase extends Element` that should be used in preference
to `@cfgnode` everywhere. This class is for exactly those use cases
where `@cfgnode` should be seen as an `Element` without having too many
member predicates on it.
The classes that move from extending `@cfgnode` to extending
`ControlFlowNodeBase` are: `BasicBlock`, `AdditionalControlFlowEdge`,
`DefOrUse`, `SsaDefinition`, `SubBasicBlock` and `RangeSsaDefinition`.
These previously had to define their own `toString` rootdef, which
typically had some dummy string as result (like `"BasicBlock"`), but now
their `toString` is part of the `Element` rootdef and should not be
overridden otherwise `Element.toString` will sometimes have multiple
results. Removing these dummy `toString` predicates had some effects on
the tests that are included in this commit.
The `getLocation` family of predicates is affected like `toString`, but
the situation is slightly different. Some of these classes had genuinely
useful alternative definitions of locations. Fortunately, they all used
`hasLocationInfo`, which is preferred over `getLocation` by the QL
engine. Because `Element` does not define `getLocationInfo`, each class
can create its own rootdef of this predicate like before.
