Some eh_frame unwind instructions will define a return address register;

when you want to find the caller's saved pc, you look up the return address
register and use that.  On arm, for instance, this would be the contents of
the link register (lr).

If the eh_frame CIE defines an RA, record that fact in the UnwindPlan.

When we're finding a saved register, if it's the pc, lok for the location
of the return address register instead.

<rdar://problem/12062310> 

llvm-svn: 162167