swift-api-digester: teach the tool to detect member variables change to global ones. rdar://41658300

test/api-digester/Outputs/apinotes-migrator-gen-revert.json

@@ -1,4 +1,15 @@
 [
+  {
+    "DiffItemKind": "CommonDiffItem",
+    "NodeKind": "Var",
+    "NodeAnnotation": "RevertSimpleStringRepresentableUpdate",
+    "ChildIndex": "0",
+    "LeftUsr": "c:@globalAttributeName",
+    "LeftComment": "AnimalAttributeName",
+    "RightUsr": "",
+    "RightComment": "String",
+    "ModuleName": "APINotesTest"
+  },
   {
     "DiffItemKind": "CommonDiffItem",
     "NodeKind": "TypeDecl",
@@ -161,6 +172,14 @@
     "NewPrintedName": "oldMember",
     "NewTypeName": "OldType"
   },
+  {
+    "DiffItemKind": "TypeMemberDiffItem",
+    "Usr": "c:@globalAttributeName",
+    "OldPrintedName": "globalAttributeName",
+    "OldTypeName": "AnimalAttributeName",
+    "NewPrintedName": "globalAttributeName",
+    "NewTypeName": ""
+  },
   {
     "DiffItemKind": "TypeMemberDiffItem",
     "Usr": "c:objc(pl)TypeWithMethod(cm)plusPrint",

tools/swift-api-digester/swift-api-digester.cpp

@@ -2543,6 +2543,12 @@ class TypeMemberDiffFinder : public SDKNodeVisitor {
     if (nodeParent->getKind() == SDKNodeKind::DeclType &&
         diffParent->getKind() == SDKNodeKind::Root)
       TypeMemberDiffs.insert({diffNode, node});
+
+    // Move from a member variable to global variable.
+    if (nodeParent->getKind() == SDKNodeKind::Root &&
+        diffParent->getKind() == SDKNodeKind::DeclType)
+      TypeMemberDiffs.insert({diffNode, node});
+
     // Move from a member variable to another member variable
     if (nodeParent->getKind() == SDKNodeKind::DeclType &&
         diffParent->getKind() == SDKNodeKind::DeclType &&
@@ -3757,7 +3763,8 @@ static void findTypeMemberDiffs(NodePtr leftSDKRoot, NodePtr rightSDKRoot,
     // index, old printed name)
     TypeMemberDiffItem item = {
         right->getAs<SDKNodeDecl>()->getUsr(),
-        rightParent->getAs<SDKNodeDecl>()->getFullyQualifiedName(),
+        rightParent->getKind() == SDKNodeKind::Root ?
+          StringRef() : rightParent->getAs<SDKNodeDecl>()->getFullyQualifiedName(),
         right->getPrintedName(), findSelfIndex(right), None,
         leftParent->getKind() == SDKNodeKind::Root ?
           StringRef() : leftParent->getAs<SDKNodeDecl>()->getFullyQualifiedName(),