Add a variant of the CommandOverrideCallback that takes a
CommandReturnObject.  Otherwise, all the overridden command
can do is say it overrode the command, not say what it did...

Also removed the duplicate definition of CommandOverrideCallback
from the private interfaces.

Now to figure out how to get this through the SB API's...

<rdar://problem/17911629>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@214938 91177308-0d34-0410-b5e6-96231b3b80d8