[SourceKit] DocInfo: Explicitly report the optionality of protocol members in response. rdar://25129725

Author: nkcsgexi

test/SourceKit/DocSupport/Inputs/cake.swift

@@ -46,4 +46,9 @@ public extension S1 {
   public struct S2 {
     public let b = 1
   }
-}
+}
+
+@objc
+public protocol P2 {
+  optional func foo1()
+}