[libclang] When determining the cursor via a location, ignore synthesized ivars otherwise

if we have something like:

   @synthesize prop = _prop;

and '_prop' is not declared, we will encounter a '_prop' ivar before
encountering the 'prop' synthesize declaration and we will think that
we passed the region-of-interest, missing the cursor for 'prop'.

rdar://12172700

llvm-svn: 162715