Fix completion after application with implicit arguments

`List(1, 2, 3).map(f).<ctrl-space>` now works; before
the tree had the type `(bf: CanBuildFrom[...]):...` and
did not contribute completions from the result type.

This commit checks if the tree has an implicit method
type, and typechecks it as a qualifier. That is enough
to get to `adaptToImplicitMethod` in the type checker,
infer the implicit arguments, and compute the final result
type accordingly.