[APINotes] Avoid duplicated attributes for class template instantiations

If a C++ class template is annotated via API Notes, the instantiations
had the attributes repeated twice. This is because Clang was adding the
attribute twice while processing the same class template. This change
makes sure we don't try to add attributes from API Notes twice.

There is currently no way to annotate specific instantiations using API
Notes.

rdar://142539959

clang/lib/Sema/SemaTemplate.cpp

@@ -2145,7 +2145,6 @@ DeclResult Sema::CheckClassTemplate(
     NewClass->startDefinition();
 
   ProcessDeclAttributeList(S, NewClass, Attr);
-  ProcessAPINotes(NewClass);
 
   if (PrevClassTemplate)
     mergeDeclAttributes(NewClass, PrevClassTemplate->getTemplatedDecl());

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

@@ -6,4 +6,5 @@ struct Box {
   const T* get_ptr() const { return &value; }
 };
 
+using FloatBox = Box<float>;
 using IntBox = Box<int>;

clang/test/APINotes/templates.cpp

@@ -7,3 +7,6 @@
 // CHECK-BOX: Dumping Box:
 // CHECK-BOX-NEXT: ClassTemplateDecl {{.+}} imported in Templates Box
 // CHECK-BOX: SwiftAttrAttr {{.+}} <<invalid sloc>> "import_owned"
+
+// Make sure the attributes aren't duplicated.
+// CHECK-BOX-NOT: SwiftAttrAttr {{.+}} <<invalid sloc>> "import_owned"