Enable ARC on the fragile runtime.

This is almost entirely a matter of just flipping a switch.  99% of
the runtime support is available all the way back to when it was
implemented in the non-fragile runtime, i.e. in Lion.  However,
fragile runtimes do not recognize ARC-style ivar layout strings,
which means that accessing __strong or __weak ivars reflectively
(e.g. via object_setIvar) will end up accessing the ivar as if it
were __unsafe_unretained.  Therefore, when using reflective
technologies like KVC, be sure that your paths always refer to a
property.

rdar://23209307

llvm-svn: 250955