SE-0364: Warning for Retroactive Conformances of External Types

[masters3d]

@lorentey

3. turning it into an error in Swift 6 mode.

It's better doing this late than never, but the cat is out of the bag now -- we have about half a decade's worth of existing binaries with such conformances that we need to keep working, and the list grows larger every day

I think a version of scoped conformance would be a better alternative

Scoped Conformances

Authors: Dave Abrahams (@dabrahams) and Dmitri Gribenko (@gribozavr), with support and feedback from the Swift for TensorFlow Team. This pitch is related to and in support of the recently posted pitch, Automatic Requirement Satisfaction in plain Swift. Introduction In Swift, protocol extensions provide a beautiful way to share API implementations across value types. For example, an extension on the Equatable protocol provides a public implementation of the != operator to all public conforming…