[4.2][CodeComplete] Typecheck SubscriptDecl context prior to completion

[rintaro]

• Explaination: Fix assertion failure when code-completing types in subscript signature. Completing signature relies on its GenericEnvironment which is set during typechecking. Previously, completing types in subscript signature regarding generic types used to hit assertion failure or just failed since declaration was not typechecked and thus GenericEnvironment was not set up.
• Scope: Changes the code-completion code for subscript signature. This change fixes assertion failure and restores expected behavior.
• Issue: rdar://problem/41227754
• Risk: Low. Affects code-completion in subscript signature only.
• Testing: Added regression test cases.
• Reviewed By: @DougGregor ([CodeComplete] Typecheck SubscriptDecl context prior to completion #17588)

---

protocol Container {
  associatedtype Element
}
struct MyStruct<T: Container> {}
extension MyStruct {
  public subscript(index: T.<HERE>) -> Int { return 0 }
  public subscript(index: Int) -> T.<HERE> { fatalError() }
}
protocol MyProtocol {
  associatedtype Content: Container
}
extension MyProtocol {
  subscript(index: Content.<HERE>) -> Int { return 0 }
  subscript(index: Int) -> Content.<HERE> { fatalError() }
}

Previously, all completion at <HERE> above used to fail (or hit assertion with assert build).

[rintaro]

@swift-ci Please test

[rintaro]

@swift-ci Please nominate