improve the diagnostic for an erroneous objc ivar reference

from:

t.m:8:7: error: member reference is not to a structure or union
  pool->farm = 0;
      ^ ~~~~
to:

t.m:8:7: error: 'NSAutoreleasePool' has member named 'farm'
  pool->farm = 0;
  ~~~~^ ~~~~

llvm-svn: 53824