Add the ability to capture the return value in a thread's stop info, and print it

as part of the thread format output.
Currently this is only done for the ThreadPlanStepOut.
Add a convenience API ABI::GetReturnValueObject.
Change the ValueObject::EvaluationPoint to BE an ExecutionContextScope, rather than
trying to hand out one of its subsidiary object's pointers.  That way this will always
be good.

llvm-svn: 146806