Demangler: fix de- and re-mangling of constrained extensions

SR-4512
rdar://problem/31471429

Author: eeckstein

lib/Demangling/Demangler.cpp

@@ -905,6 +905,10 @@ NodePointer Demangler::demangleBoundGenericArgs(NodePointer Nominal,
       BoundParent = createWithChildren(Node::Kind::Extension,
                                        Context->getFirstChild(),
                                        BoundParent);
+      if (Context->getNumChildren() == 3) {
+        // Add the generic signature of the extension context.
+        addChild(BoundParent, Context->getChild(2));
+      }
     } else {
       BoundParent = demangleBoundGenericArgs(Context, TypeLists, TypeListIdx);
     }

lib/Demangling/Remangler.cpp

@@ -1784,6 +1784,10 @@ NodePointer Demangle::getUnspecialized(Node *node, NodeFactory &Factory) {
       NodePointer result = Factory.createNode(Node::Kind::Extension);
       result->addChild(node->getFirstChild(), Factory);
       result->addChild(getUnspecialized(parent, Factory), Factory);
+      if (node->getNumChildren() == 3) {
+        // Add the generic signature of the extension.
+        result->addChild(node->getChild(2), Factory);
+      }
       return result;
     }
     default:

test/Demangle/Inputs/manglings.txt

@@ -248,4 +248,5 @@ _SocketJoinOrLeaveMulticast ---> _SocketJoinOrLeaveMulticast
 _T0s10DictionaryV3t17E6Index2V1loiSbAEyxq__G_AGtFZ ---> static (extension in t17):Swift.Dictionary.Index2.< infix ((extension in t17):[A : B].Index2, (extension in t17):[A : B].Index2) -> Swift.Bool
 _T08mangling14varargsVsArrayySaySiG3arrd_SS1ntF ---> mangling.varargsVsArray (arr : Swift.Int..., n : Swift.String) -> ()
 _T08mangling14varargsVsArrayySaySiG3arrd_tF ---> mangling.varargsVsArray (arr : Swift.Int...) -> ()
-                                                                                                                           
+_T0s13_UnicodeViewsVss22RandomAccessCollectionRzs0A8EncodingR_11SubSequence_5IndexQZAFRtzsAcERpzAE_AEQZAIRSs15UnsignedInteger8Iterator_7ElementRPzAE_AlMQZANRS13EncodedScalar_AlMQY_AORSr0_lE13CharacterViewVyxq__G ---> (extension in Swift):Swift._UnicodeViews<A, B><A, B where A: Swift.RandomAccessCollection, B: Swift.UnicodeEncoding, A.Index == A.SubSequence.Index, A.SubSequence: Swift.RandomAccessCollection, A.SubSequence == A.SubSequence.SubSequence, A.Iterator.Element: Swift.UnsignedInteger, A.Iterator.Element == A.SubSequence.Iterator.Element, A.SubSequence.Iterator.Element == B.EncodedScalar.Iterator.Element>.CharacterView
+