[Sema] Allow extending specialized types

[Azoy]

While we patiently wait for parameterized extensions, I figured this feature can be split off and proposed separately so that people can use some much needed sugar for extensions. This allows for the following:

extension Array<Int> {} // translates to `extension Array where Element == Int {}`

This means that users can now finally use type sugar for their extended types like the following:

extension [Character] {
  func makeString() -> String {
    // ...
  }
}

In the process of doing this, I stumbled upon https://bugs.swift.org/browse/SR-4875 , and removing the constrained extension diagnostic means that this bug should probably just be fixed, so it is now.

Resolves: https://bugs.swift.org/browse/SR-4875

[Azoy]

@swift-ci please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 418af510ffedeee9eeffab4efa732168f46d1fcd

[CodaFi]

@swift-ci clean test Linux

[CodaFi]

Looks like requirement_inference.swift needs updating as well.

[CodaFi]

Brilliant 👍

Does this need to go through evolution?

[AnthonyLatsis]

🎉

Does this need to go through evolution?

Speaking of which, I remember some people were sceptic about this because it wasn't obvious to them whether Array<Class> would translate to == or :.

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 418af510ffedeee9eeffab4efa732168f46d1fcd

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 418af510ffedeee9eeffab4efa732168f46d1fcd

[Azoy]

@swift-ci please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - db2f077

[Azoy]

@swift-ci clean test Linux