[SourceKit][DocInfo] When the underlying type of a type alias decl exists, report the conformances of the underlying type. rdar://26408167 (#3394)

Showing only the conforming associated types provides
little information to doc viewers. This patch digs the
underlying type of an associated type to report the
conformance info of those.

Author: nkcsgexi

test/SourceKit/DocSupport/Inputs/cake.swift

@@ -52,3 +52,11 @@ public extension S1 {
 public protocol P2 {
   @objc optional func foo1()
 }
+
+public protocol P3 {
+  associatedtype T
+}
+
+public struct S2 : P3 {
+  public typealias T = S2
+}

test/SourceKit/DocSupport/doc_clang_module.swift.response

@@ -6049,7 +6049,24 @@ var FooSubUnnamedEnumeratorA1: Int { get }
     key.doc.full_as_xml: "<Typedef file=Foo.h line=\"59\" column=\"13\"><Name>FooTypedef1</Name><USR>c:Foo.h@T@FooTypedef1</USR><Declaration>typealias FooTypedef1 = Int32</Declaration><Abstract><Para> Aaa.  FooTypedef1.  Bbb.</Para></Abstract></Typedef>",
     key.offset: 3028,
     key.length: 29,
-    key.fully_annotated_decl: "<decl.typealias><syntaxtype.keyword>typealias</syntaxtype.keyword> <decl.name>FooTypedef1</decl.name> = <ref.struct usr=\"s:Vs5Int32\">Int32</ref.struct></decl.typealias>"
+    key.fully_annotated_decl: "<decl.typealias><syntaxtype.keyword>typealias</syntaxtype.keyword> <decl.name>FooTypedef1</decl.name> = <ref.struct usr=\"s:Vs5Int32\">Int32</ref.struct></decl.typealias>",
+    key.conforms: [
+      {
+        key.kind: source.lang.swift.ref.protocol,
+        key.name: "SignedInteger",
+        key.usr: "s:Ps13SignedInteger"
+      },
+      {
+        key.kind: source.lang.swift.ref.protocol,
+        key.name: "Comparable",
+        key.usr: "s:Ps10Comparable"
+      },
+      {
+        key.kind: source.lang.swift.ref.protocol,
+        key.name: "Equatable",
+        key.usr: "s:Ps9Equatable"
+      }
+    ]
   },
   {
     key.kind: source.lang.swift.decl.var.global,

test/SourceKit/DocSupport/doc_source_file.swift.response

@@ -2586,9 +2586,19 @@
         key.fully_annotated_decl: "<decl.typealias><syntaxtype.keyword>typealias</syntaxtype.keyword> <decl.name>Element</decl.name> = <ref.struct usr=\"s:Si\">Int</ref.struct></decl.typealias>",
         key.conforms: [
           {
-            key.kind: source.lang.swift.ref.associatedtype,
-            key.name: "Element",
-            key.usr: "s:P8__main__5Prot27Element"
+            key.kind: source.lang.swift.ref.protocol,
+            key.name: "SignedInteger",
+            key.usr: "s:Ps13SignedInteger"
+          },
+          {
+            key.kind: source.lang.swift.ref.protocol,
+            key.name: "Comparable",
+            key.usr: "s:Ps10Comparable"
+          },
+          {
+            key.kind: source.lang.swift.ref.protocol,
+            key.name: "Equatable",
+            key.usr: "s:Ps9Equatable"
           }
         ]
       },