Add a comment mirroring the one in LLVM's Dwarf.h

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@275356 91177308-0d34-0410-b5e6-96231b3b80d8

include/clang/Sema/DeclSpec.h

@@ -793,6 +793,7 @@ public:
   };
 
   /// PropertyAttributeKind - list of property attributes.
+  /// Keep this list in sync with LLVM's Dwarf.h ApplePropertyAttributes.
   enum ObjCPropertyAttributeKind {
     DQ_PR_noattr = 0x0,
     DQ_PR_readonly = 0x01,