objective-c arc: ARC IRGen correctly assumes result

type of generated call to super dealloc is 'void'
and asserts if user's dealloc is not of 'void type.
This rule must be enforced in clang front-end (with a 
fixit) if this is not the case, instead of asserting in CodeGen.
// rdar://11987838

llvm-svn: 160993