[clang] ThreadSafetyAnalysis.rst: do not re-define THREAD_ANNOTATION_ATTRIBUTE__ macro

[lebedev.ri]

Summary

When trying to actually use that example header, with gcc compiler, i got a warning that THREAD_ANNOTATION_ATTRIBUTE__ macro is being re-defined.
Which makes sense, since that macro was just defined on previous line[s]

[delesley]

LOL. Thanks!

[aaron.ballman]

LGTM!

[lebedev.ri]

Thank you.

No commit rights here, so someone else will have to commit.

[aaron.ballman]

Thanks! I've commit in r302275.

docs/ThreadSafetyAnalysis.rst

.. code-block:: c++
   // Enable thread safety attributes only with clang.
   // The attributes can be safely erased when compiling with other compilers.
   #if defined(__clang__) && (!defined(SWIG))
   #define THREAD_ANNOTATION_ATTRIBUTE__(x)   __attribute__((x))
   #else
   #define THREAD_ANNOTATION_ATTRIBUTE__(x)   // no-op
   #endif
 
-  #define THREAD_ANNOTATION_ATTRIBUTE__(x)   __attribute__((x))
-
   #define CAPABILITY(x) \
     THREAD_ANNOTATION_ATTRIBUTE__(capability(x))
 
   #define SCOPED_CAPABILITY \
     THREAD_ANNOTATION_ATTRIBUTE__(scoped_lockable)
 
   #define GUARDED_BY(x) \
     THREAD_ANNOTATION_ATTRIBUTE__(guarded_by(x))