[SourceKit/DocSupport] List generic requirements from contextual where clauses in doc info request

We previously didn't report the requirements in the where clause of 'boxes'
below because it didn't have generic parameters of its own:

public struct Box<Wrapped> {
    public func boxes() -> [Box<Wrapped.Element>] where Wrapped: Sequence { fatalError() }
}

Resolves rdar://problem/60658263

Author: Nathan Hawes

test/SourceKit/DocSupport/Inputs/cake.swift

@@ -106,3 +106,16 @@ some comments
 public extension C1 {
   func addition() {}
 }
+
+
+public struct Box<Wrapped> {
+    public func boxes() -> [Box<Wrapped.Element>] where Wrapped: Sequence { fatalError() }
+}
+
+public protocol P {
+    func foo()
+}
+
+public extension P {
+    func bar() where Self: Equatable {}
+}