Removing the alloc_size attribute. The attribute was semantically handled, but silently ignored. Most of this feature was already reverted in June 2012 (r159016), this just cleans up the pieces left over.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@197866 91177308-0d34-0410-b5e6-96231b3b80d8