Add support for objc_unsafeClaimAutoreleasedReturnValue to the

ObjC ARC Optimizer.

The main implication of this is:

1. Ensuring that we treat it conservatively in terms of optimization.
2. We put the ASM marker on it so that the runtime can recognize
objc_unsafeClaimAutoreleasedReturnValue from releaseRV.

<rdar://problem/21567064>

Patch by Michael Gottesman!

llvm-svn: 258970