Interpolate any type vars from comparing against SelectionProto

[dwijnand]

If the method being selected is polymorphic, then it is added, along
with fresh type vars, to the constraint. Normally this is fine, because
when the selection is then applied, the type vars are interpolated
(during tree simplification). But if the method being selected is a
macro that's being jointly compiled, then the compilation may be
suspended, which leaves a trailing type lambda in the constraint.

I tried addressing this in a few different ways. Initially with an
exemption in TreeChecker, but that seemed too remote and too liberal.
Later I tried addressing it specifically in inlining (in inlineIfNeeded)
but that also seemed too remote and extensive. In the end I thought
handling this around the testSubType call seemed most suitable.
Interpolating on all the new type vars in general is too eager, so I
ended up guarding it only for SelectionProto, sadly.

[dwijnand]

@smarter Any thoughts here? I'm not sure what approach to take, or whether anyone cares about the issue.

[smarter]

I'm not sure either, would replacing the super.typedXX(...) calls by super.typed(...) calls to include the simplify step work?

[dwijnand]

No, I fairly sure not, because that would loop around between InlineTyper.typedApply and super.typed..

I think the architectural way to do this might be to shuffle the logic that InlineTyper is doing into its adapt post-processing, which is post-simplify - but that seemed like a bigger change that I would want to double-check first.

[smarter]

Let's go with this for now then.