Use the generalized implied result logic, and
rename to `isImpliedResult` since that's really
what we're querying here, and it needs to handle
implicit-last-exprs if enabled.
This will allow us to run two different completion kinds and deliver results from both of them.
Also: Compute a unified type context for global lookup. Previously, we always used the expected type context of the last lookup. But really, we should be considering all possible types from all constraint system solutions when computing code completion results from the cache.
This eliminates a source of bugs if subclasses of `TypeCheckCompletionCallback` forget to call the superclass’s implementation of `sawSolution` from their overridden method.
