[Typechecker] Relax existential to concrete type cast check for non-final classes

[theblixguy]

Relax existential to concrete type cast check for classes, otherwise we have emit a warning for the code below:

protocol MyProtocol {}
class Superclass {} // Superclass does not conform to MyProtocol
class Subclass: Superclass, MyProtocol {} // Subclass conforms to MyProtocol

func f(_ x: MyProtocol) { // Expects existential of type MyProtocol
  if let y = x as? Superclass { // Superclass is 'unrelated', but cast still succeeds
    print(y)
  }
}

f(Subclass())

Resolves SR-11402.

[harlanhaskins]

LGTM, thanks for the quick fix @theblixguy!

[theblixguy]

@swift-ci please smoke test

[theblixguy]

@swift-ci please smoke test

[theblixguy]

@swift-ci please smoke test

[theblixguy]

@swift-ci please smoke test

[harlanhaskins]

This LGTM! Thanks again @theblixguy!

[xedin]

Perfect, thank you!