Implementation "step out" plans shouldn't gather the return value.

When, for instance, "step-in" steps into a function that it doesn't want
to stop in (e.g. has no debug info) it will push a step-out plan to implement
the step out so it can then continue stepping.  These step out's don't use
the result of the function stepped out of, so they shouldn't spend the time 
to compute it.

llvm-svn: 279540