Sema: Ban unavailable overrides of available decls

[tshortli]

Unavailable decls should not be allowed to override available decls since overrides can be invoked at runtime through vtables and are therefore reachable at runtime if the base declaration is available. For example:

@available(*, unavailable)
struct Unavailable {
  init() { print("Oops") }
}

class Base {
  required init() {}

  class func factory() -> Self {
    Self.init()
  }
}

class Derived: Base {
  @available(*, unavailable)
  required init() {
    super.init()
    _ = Unavailable()
  }
}

_ = Derived.factory() // Oops

Resolves rdar://108396778

[tshortli]

@swift-ci please test

[tshortli]

@swift-ci please test source compatibility

[tshortli]

It looks like banning is going to be too much of a source break. In that case, I think we should probably make it behave the way developers would intuitively think it would behave and instead remove unavailable methods from vtables so that they don't get executed via dynamic dispatch.

[tshortli]

I turned the diagnostic improvements and test coverage improvements from this change into a separate PR that doesn't restrict unavailable decls from overriding available decls: #65456

[tshortli]

Closing this PR.