[5.5][Sema] Fix crash when retrieving typeContextInfo for a partially bound generic type

[ahoppen]

Cherry-picks #37034.

---

In the following test case, we are crashing while building the generic signature of someGenericFunc, potentially invoked on model in line 11.

struct MyBinding<BindingOuter> {
  func someGenericFunc<BindingInner>(x: BindingInner) {}
}

struct MyTextField<TextFieldOuter> {
  init<TextFieldInner>(text: MyBinding<TextFieldInner>) {}
}

struct EncodedView {
    func foo(model: MyBinding<String>) {
        let _ = MyTextField<String>(text: model)
    }
}

Because we know that model has type MyBinding<TextFieldInner>, we substitute the BindingOuter generic parameter by TextFieldInner. Thus, someGenericFunc has the signature <TextFieldInner /* substitutes BindingOuter */, BindingInner>. TextFieldInner and BindingOuter both have depth = 1, index = 0. Thus the verification in GenericSignatureBuilder is failing.

After discussion with Slava, the root issue appears to be that we shouldn’t be calling subst on a GenericFunctionType at all. Instead we should be using substGenericArgs which doesn’t attempt to rebuild a generic signature, but instead builds a non-generic function type.

---

We slightly regress in code completion results by showing two collidingGeneric twice in the following case.

protocol P1 {
  func collidingGeneric<T>(x: T)
}
protocol P2 {
  func collidingGeneric<T>(x: T)
}
class C : P1, P2 {
  #^COMPLETE^#
}

Previously, we were representing the type of collidingGeneric by a generic function type with generic param T that doesn’t have any restrictions. Since we are now using substGenericArgs instead of subst, we receive a non-generic function type that represents T as an archetype. And since that archetype is different for the two function signatures, we show the result twice in code completion.

One could also argue that showing the result twice is intended (or at least acceptable) behaviour since, the two protocol may name their generic params differently. E.g. in

protocol P1 {
  func collidingGeneric<S>(x: S)
}
protocol P2 {
  func collidingGeneric<T>(x: T)
}
class C : P1, P2 {
  #^COMPLETE^#
}

we might be expected to show the following two results

func collidingGeneric<S>(x: S)
func collidingGeneric<T>(x: T)

[ahoppen]

@swift-ci Please test

[swift-ci]

Build failed
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Git Sha - 80fa362

[ahoppen]

@swift-ci Please test Linux