Sema: Ban unsound convenience initializer inheritance inside a module

[slavapestov]

We already prevent the user from subclassing a class with an inaccessible
designated initializer from another module,

open class C {
  private init() {}
  public convenience init(x: Int) { self.init() }
}

However, we should ban this within a module too:

class C {
  private init() {}
  convenience init(x: Int) { self.init() }
}

class D: C {}

Today, we don't diagnose anything, and we also don't synthesize the
override, so we just miscompile instead.

We can't synthesize the override though, because this would then
allow calling the superclass's private init from the subclass, which
violates access control.

So the only option is to ban this.

Fixes rdar://68220237.
Fixes #61508.

[slavapestov]

@swift-ci Please smoke test

[slavapestov]

@swift-ci Please test source compatibility

[allevato]

Out of curiosity, is there anything still blocking this fix?

I happened to find my way here after one of our teams put themselves into a similar situation—they refactored a class so that it only contained a private designated initializer, so I suggested making the class final, and then went to Godbolt because I was wondering "what if..."