AssociatedTypeInference: Allow Self as a fixed type witness

[AnthonyLatsis]

To gain more control over what we reject in computeFixedTypeWitness, pull the nested type out of the Self archetype rather than asking the generic signature for a concrete type.

[slavapestov]

How about mapTypeIntoContet(assocType->getDeclaredInterfaceType())

[slavapestov]

... oops, I missed the mapTypeOutOfContext(). I think this is easier to accomplish using genericSig->getCanonicalTypeInContext()

[AnthonyLatsis]

Currently, we only look through protocols that inherit the protocol of the given associated type, but I was going to remove that limitation eventually, because a fixed type witness can come from an unrelated protocol with a matching associated type:

protocol P {
  associatedtype A
}
protocol P2: P where A == Int {}
protocol Q {
  associatedtype A
}
struct Foo: Q, P2 {} // error while checking conformance to Q

The problem with getCanonicalTypeInContext is that it fails when given a foreign dependent member, so I decided to build on the name of the associated type to avoid reimplementing this later.

[slavapestov]

What if you use DependentMemberType::get() to get the associated type-less form, using just an identifier? It would still avoid mapping in and out of context.

[AnthonyLatsis]

Cool, thanks!

[AnthonyLatsis]

@swift-ci please smoke test

[AnthonyLatsis]

@slavapestov This is ready to go as well, I suppose?

[AnthonyLatsis]

@slavapestov ping