[API Notes] Ensure that redeclaration of tags doesn't duplicate attributes

(cherry picked from commit 6e2683c)

Author: DougGregor

clang/test/APINotes/Inputs/Headers/SwiftImportAs.h

@@ -20,8 +20,8 @@ struct CopyableType { int value; };
 struct NonEscapableType { int value; };
 struct EscapableType { int value; };
 
-
 struct OpaqueRefCountedType;
+struct OpaqueRefCountedType; // redeclaration
 
 inline void ORCRetain(struct OpaqueRefCountedType *x);
 inline void ORCRelease(struct OpaqueRefCountedType *x);

clang/test/APINotes/swift-import-as.cpp

@@ -32,7 +32,15 @@
 // CHECK-OPAQUE-REF-COUNTED: SwiftAttrAttr {{.+}} <<invalid sloc>> "import_reference"
 // CHECK-OPAQUE-REF-COUNTED: SwiftAttrAttr {{.+}} <<invalid sloc>> "retain:ORCRetain"
 // CHECK-OPAQUE-REF-COUNTED: SwiftAttrAttr {{.+}} <<invalid sloc>> "release:ORCRelease"
+// CHECK-OPAQUE-REF-COUNTED-NOT: SwiftAttrAttr {{.+}} <<invalid sloc>> "release:ORCRelease"
 
+// CHECK-OPAQUE-REF-COUNTED: Dumping OpaqueRefCountedType:
+// CHECK-OPAQUE-REF-COUNTED-NEXT: CXXRecordDecl {{.+}} imported in SwiftImportAs{{.*}}struct OpaqueRefCountedType
+// CHECK-OPAQUE-REF-COUNTED: SwiftAttrAttr {{.+}} <<invalid sloc>> "import_reference"
+// CHECK-OPAQUE-REF-COUNTED: SwiftAttrAttr {{.+}} <<invalid sloc>> "retain:ORCRetain"
+// CHECK-OPAQUE-REF-COUNTED: SwiftAttrAttr {{.+}} <<invalid sloc>> "release:ORCRelease"
+
+// CHECK-OPAQUE-REF-COUNTED-NOT: SwiftAttrAttr {{.+}} <<invalid sloc>> "release:
 // CHECK-NON-COPYABLE: Dumping NonCopyableType:
 // CHECK-NON-COPYABLE-NEXT: CXXRecordDecl {{.+}} imported in SwiftImportAs {{.+}} struct NonCopyableType
 // CHECK-NON-COPYABLE: SwiftAttrAttr {{.+}} <<invalid sloc>> "conforms_to:MySwiftModule.MySwiftNonCopyableProtocol"