[arcmt] Emit an error for unused -autorelease messages.

[arcmt] Emit an error for unused -autorelease messages.

An unused autorelease is badness. If we remove it the receiver
will likely die immediately while previously it was kept alive
by the autorelease pool. This is bad practice in general, so leave it
and emit an error to force the user to restructure his code.

rdar://9599884

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@135193 91177308-0d34-0410-b5e6-96231b3b80d8