swift-module-digester: fix a false positive when diagnosing super class changes.

Author: nkcsgexi

test/api-digester/Inputs/cake1.swift

@@ -94,3 +94,5 @@ public class SuperClassRemoval: C3 {}
 
 public class ClassToStruct {}
 public protocol ProtocolToEnum {}
+
+public class SuperClassChange: C7 {}

test/api-digester/Inputs/cake2.swift

@@ -71,6 +71,8 @@ public class C7: P1 {
   public func foo(_ a: Int, _ b: Int) {}
 }
 
+public class C8: C7 {}
+
 public protocol P3: P1, P4 {}
 
 public protocol P4 {}
@@ -101,3 +103,5 @@ public class SuperClassRemoval {}
 
 public struct ClassToStruct {}
 public enum ProtocolToEnum {}
+
+public class SuperClassChange: C8 {}

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

@@ -710,9 +710,7 @@ static void diagnoseNominalTypeDeclChange(SDKNodeDeclType *L, SDKNodeDeclType *R
       if (RSuperClass.empty()) {
         Diags.diagnose(SourceLoc(), diag::super_class_removed, L->getScreenInfo(),
                        LSuperClass);
-      } else {
-        // FIXME: This will be a false positive if the new subclass is a subclass
-        // of the old type.
+      } else if (!contains(R->getClassInheritanceChain(), LSuperClass)) {
         Diags.diagnose(SourceLoc(), diag::super_class_changed, L->getScreenInfo(),
                        LSuperClass, RSuperClass);
       }