[4.0][migrator] When renaming a function decl, we should use underscore to represent empty external argument label. rdar://34569243

lib/Migrator/APIDiffMigratorPass.cpp

@@ -684,14 +684,19 @@ struct APIDiffMigratorPass : public ASTMigratorPass, public SourceEntityWalker {
           StringRef NewArg = View.args()[Index++];
           auto ArgLoc = PD->getArgumentNameLoc();
 
+          // Represent empty label with underscore.
+          if (NewArg.empty())
+            NewArg = "_";
+
           // If the argument name is not specified, add the argument name before
           // the parameter name.
           if (ArgLoc.isInvalid())
             Editor.insertBefore(PD->getNameLoc(),
                                 (llvm::Twine(NewArg) + " ").str());
-          else
+          else {
             // Otherwise, replace the argument name directly.
             Editor.replaceToken(ArgLoc, NewArg);
+          }
         }
       }
     }

test/Migrator/Inputs/API.json

@@ -446,6 +446,17 @@
     "RightComment": "setZooLocationNew(newX:newY:newZ:)",
     "ModuleName": "Cities"
   },
+  {
+    "DiffItemKind": "CommonDiffItem",
+    "NodeKind": "Function",
+    "NodeAnnotation": "Rename",
+    "ChildIndex": "0",
+    "LeftUsr": "s:6Cities04MoreA0P8addZooAtySi_Si1ySi1ztF",
+    "LeftComment": "",
+    "RightUsr": "",
+    "RightComment": "addNewZooAt(_:newY:newZ:)",
+    "ModuleName": "Cities"
+  },
   {
     "DiffItemKind": "CommonDiffItem",
     "NodeKind": "Function",

test/Migrator/Inputs/Cities.swift

@@ -22,6 +22,7 @@ public protocol ExtraCities {
 
 public protocol MoreCities {
   func setZooLocation(x: Int, y: Int, z: Int)
+  func addZooAt(_ x: Int, y: Int, z: Int)
 }
 
 public func setCityProperty1(_ c : Cities, _ p : Int) {}

test/Migrator/rename-func-decl.swift

@@ -8,4 +8,5 @@ import Cities
 
 class MyCities : MoreCities {
   func setZooLocation(x ix: Int, y iy: Int, z iz: Int) {}
+  func addZooAt(_ x: Int, y: Int, z: Int) {}
 }

test/Migrator/rename-func-decl.swift.expected

@@ -8,4 +8,5 @@ import Cities
 
 class MyCities : MoreCities {
   func setZooLocationNew(newX ix: Int, newY iy: Int, newZ iz: Int) {}
+  func addNewZooAt(_ x: Int, newY y: Int, newZ z: Int) {}
 }