[ConstraintSystem] Fix crash on function conversion reliant on conditional conformance

[theblixguy]

This PR fixes a compiler crasher related to a function conversion reliant on conditional conformance. Example:

protocol P {}
struct S<T> {}
extension S : P where T : P {}

func foo(_ fn: (S<String>) -> Void) {}
func bar(_ fn: (P) -> Void) {
  foo(fn)
}

Resolves SR-10992.

[theblixguy]

cc @xedin I have added a new overload for getConstraintLocator based on your suggestion!

[xedin]

@theblixguy Could you please convert example from validation test to the one which checks produces diagnostic? Also I think there are probably more places which could benefit from using this new method, at least CSDiagnostics probably should.

How about adding new getConstraintLocator method to FailureDiagnostic itself to avoid unnecessary indirection?

[theblixguy]

@xedin Do you mean you want the exact same method in FailureDiagnostic as well (which would then internally call CS.getConstraintLocator(anchor, path, flags))?

I kept it in CS so the declaration stays in one place (instead of multiple copies).

[xedin]

@theblixguy Yes, I think it could be done separately but it would make sense to call it on FailureDiagnostic instead of current indirect way. At least it would be great to audit where summaryFlags is set to 0 at the moment and change that here.

[theblixguy]

@xedin Okay, I have added it to FailureDiagnostic! Found two more uses of summaryFlags being 0 which I removed.

[xedin]

@swift-ci please test

[theblixguy]

All tests have passed! I'll cherry pick this to 5.1 if that's okay?

[xedin]

@theblixguy Sounds good, thanks!