Issue Description:

Only seems to reproduce when split across files:

{code: title=$ cat /tmp/first.swift}
protocol ChatDataSourceDelegateProtocol {
func chatDataSourceDidUpdate()
}

class BaseChatViewController {
var foo = 1
func bar() {
print(self . /cursor-info->/foo)
}
}

{code: title=$ cat /tmp/second.swift}
extension BaseChatViewController: ChatDataSourceDelegateProtocol {
 func chatDataSourceDidUpdate() \{ fatalError() }
}

{code: title=$ sourcekitd-test -req=cursor -pos=8:35 /tmp/first.swift – /tmp/first.swift /tmp/second.swift}
{
key.request: source.request.cursorinfo,
key.compilerargs: [
"/tmp/first.swift",
"/tmp/second.swift"
],
key.offset: 180,
key.sourcefile: "/tmp/first.swift"
}
error response (Connection Interrupted): Connection interrupted
sourcekit: [1:connection-event-handler:4099: 0.0000] Connection interruptsourcekit: [1:updateSemanticEditorDelay:4099: 0.0019] disabling semantic editor for 10 secondssourcekit: [1:pingService:4099: 0.0019] pinging serviceerror response (Connection Interrupted): Connection interrupted