Rearrange RegisterContextLLDB::SavedLocationForRegister a tiny bit

so that we will use the UnwindPlan's rule for providing the stack
pointer BEFORE we use the trick of using the callee's CFA address
as the stack pointer.  When we're in a _sigtramp frame, the CFA of
the _sigtramp stack frame is not the same as the stack pointer value
when the async interrupt occurred -- we need to use the eh_frame
rules for retrieving the correct value.

<rdar://problem/18913548> 

llvm-svn: 225427