[arcmt] Remove an unused -autorelease, without failing with error, for this

idiom that is used commonly in setters:

   [backingValue autorelease];
   backingValue = [newValue retain]; // in general a +1 assign

rdar://9914061

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@157347 91177308-0d34-0410-b5e6-96231b3b80d8