SI-7548 askTypeAt returns the same type whether the source was fully or targeted type-checked

When asking for targeted typecheck, the located tree may have overloaded types
is the source isn't yet fully typechecked (e.g., a select tree for an
overloaded method). This is problematic as it can lead to unknown 'hovers',
broken hyperlinking that suddenly starts working, unresolved ScalaDoc comments,
and similar, in the Scala IDE.

With this commit we are hardening the contract of `askTypeAt` to return the
same type whether the file was fully type-checked or targeted type-checked.
This is done by preventing the typechecker to stop too early if the `located`
tree has an overloaded type. Furthermore, I'm assuming that if `located.tpe`
is of type `OverloadedType`, by letting the compiler carry-on the typechecking,
the `located.tpe` will eventually be resolved to a non-overloaded type. Said
otherwise, I expect the targeted typechecking will always terminate (if my
reasoning isn't sound, please say so).

The test provided with this commit demonstrates the new behavior (the position
used to execute the test is resolved to the `foo` method's call). In fact,
before this commit, executing the test returned the following:

    (x: Int, y: String)Unit <and> (x: String)Unit <and> (x: Int)Unit

Showing that the tree's type is an overloaded type. The ambiguity is fixed by
this commit, and in fact the test's output is now:

    (x: Int)Unit