[IDE/SourceKit] New SourceKit request for filtered method list

`source.request.conformingmethods` is a new SourceKit request which
receives a source position and a list of protocol namses, returns a list
of methods whose return type conforms to the requested protocols.

rdar://problem/44699573

test/SourceKit/ConformingMethods/basic.swift

@@ -0,0 +1,30 @@
+protocol Target1 {}
+protocol Target2 {}
+protocol Target3 {}
+
+struct ConcreteTarget1 : Target1 {}
+struct ConcreteTarget2 : Target2 {}
+struct ConcreteTarget3 : Target3 {}
+
+protocol P {
+  associatedtype Assoc
+  func protocolMethod(asc: Assoc) -> Self
+}
+extension P {
+  func protocolMethod(asc: Assoc) -> Self { return self }
+}
+
+class C : P {
+  typealias Assoc = String
+  static func staticMethod() -> Self {}
+  func instanceMethod(x: Int) -> C {}
+  func methodForTarget1() -> ConcreteTarget1 {}
+  func methodForTarget2() -> ConcreteTarget2 {}
+}
+
+func testing(obj: C) {
+  let _ = obj.
+}
+
+// RUN: %sourcekitd-test -req=conformingmethods -pos=26:14 %s -req-opts=expectedtypes='$s8MyModule7Target2PD;$s8MyModule7Target1PD' -- -module-name MyModule %s > %t.response
+// RUN: diff -u %s.response %t.response