[CSDiagnostics] Offer protocol conformance fix-it in ContextualFailure

[theblixguy]

If we're assigning a value of concrete type to a protocol type but the type does not conform to the protocol and it's the same as the decl context's type, then offer a fix-it to add the conformance.

For example:

protocol Foo {}

class Bar1 {
  var prop: Foo?
}

class Bar2 {
  let bar1 = Bar1()
  
  func method() {
    bar1.prop = self // offer fix-it to conform 'Bar2' to 'Foo'
  }
}

Resolves SR-11412.

[theblixguy]

This is my initial implementation, but let me know your thoughts :)

[owenv]

It's nice to see this get picked up so quickly!

This isn't a full review, just had a couple of comments.

Also, what do you think about offering to add requirement stubs when in editor mode? As is, applying this fix-it will usually result in the 'missing requirements' error and the fix-it to add stubs.

[xedin]

I think this is a great idea!

[theblixguy]

@swift-ci please smoke test