[CSDiagnostics] Emit fix-its to insert requirement stubs for missing protocols in context

[theblixguy]

This is a follow-up to #27026.

---

At the moment, we emit a fix-it to insert the missing conformance. For example:

protocol P {
  func method()
}

class C {
  var p: P?
  
  func assign() {
    p = self
  }
}

Here, we emit fix-it to add : P to C. With this change, we now also emit the requirement stub (func method()) if we're in editor mode.

[hborla]

Will this still add the requirement stub if the type already implements the requirement?

[theblixguy]

Yeah, it will, which is unfortunate. I am assuming that in most cases there won't be any conflicts. There is filterProtocolRequirements which seems to remove conflicting declarations. Let me give that a try.

[theblixguy]

Oh never mind, I misunderstood the role of filterProtocolRequirements (it filters conflicting requirements, it doesn't filter requirements that are already implemented).

I figured out another way using ConformanceChecker which seems to work. Let me know what you think of this approach.

[xedin]

@swift-ci please build toolchain macOS

[theblixguy]

Toolchain: https://ci.swift.org/job/swift-PR-toolchain-osx/641/artifact/branch-master/swift-PR-33628-641-osx.tar.gz

[xedin]

LGTM! Just wanted to give it a try and see where it would insert these stubs

[theblixguy]

@swift-ci please smoke test

[theblixguy]

@swift-ci please smoke test macOS