message: Remove an erronous __attribute__((...)) tag.
  There is no __attribute__((guarded)) or __attribute__((pt_guarded)) attribute in Clang, and if we turn on warnings for unknown attributes (which are currently turned off), this causes build failures.  This means that, when the warnings are turned off, this is simply a no-op.
diff: @@ -50,7 +50,7 @@ limitations under the License.
  // a shared variable is guarded by some unspecified mutex, for use in rare
  // cases where a valid mutex expression cannot be specified.
  #define GUARDED_BY(x) THREAD_ANNOTATION_ATTRIBUTE__(guarded_by(x))
  -#define GUARDED_VAR THREAD_ANNOTATION_ATTRIBUTE__(guarded)
  +#define GUARDED_VAR // no-op
  // Document if the memory location pointed to by a pointer should be guarded
  // by a mutex when dereferencing the pointer. PT_GUARDED_VAR is analogous to
  @@ -60,7 +60,7 @@ limitations under the License.
  // guarded by mu2, q should be annotated as follows:
  // int *q GUARDED_BY(mu1) PT_GUARDED_BY(mu2);
  #define PT_GUARDED_BY(x) THREAD_ANNOTATION_ATTRIBUTE__(pt_guarded_by(x))
  -#define PT_GUARDED_VAR THREAD_ANNOTATION_ATTRIBUTE__(pt_guarded)
  +#define PT_GUARDED_VAR // no-op
  // Document the acquisition order between locks that can be held
  // simultaneously by a thread. For any two locks that need to be annotated
  