Non optimizable objc_retainBlock calls are not forwarding.

Since we handle optimizable objc_retainBlocks through strength reduction
in OptimizableIndividualCalls, we know that all code after that point
will only see non-optimizable objc_retainBlock calls. IsForwarding is
only called by functions after that point, so it is ok to just classify
objc_retainBlock as non-forwarding.

<rdar://problem/13249661>.

llvm-svn: 178285