[SR-4744] @objc Attribute validation should support Protocol Extension declaration

[swift-ci]

Previous ID	SR-4744
Radar	None
Original Reporter	garricn (JIRA User)
Type	Improvement
Status	Resolved
Resolution	Won't Do

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Improvement, Runtime
Assignee	None
Priority	Medium

md5: f63bdf0f8e3527e2918ab4f55f8dc55b

duplicates:

• SR-3349 @objc methods cannot be contained in protocol extensions

Issue Description:

According to https://twitter.com/garricn/status/858489431270936577 the following code should compile:
Apparently, this is a known issue:

import Foundation

@objc protocol SomeProtocol {}

extension SomeProtocol {
    @objc func someFunction() {}
}

However, this code produces the following error at the second use of @objc:

@objc can only be used with members of classes, @objc protocols, and concrete extensions of classes

[belkadan]

See SR-3349 for discussion.