Fix a glitch in the Driver's batch mode when used with "attach".

Batch mode is supposed to stop execution and return control to the user when an
exceptional stop occurs (crash, signal or instrumentation).  But attach always stops
with a SIGSTOP on OSX (maybe on Linux too?) which would short circuit the rest of the
commands given.

This change allows a command result object to indicate that it expected to leave the 
process stopped with an exceptional stop reason, and it is okay for batch mode to keep going.

<rdar://problem/22243143>

llvm-svn: 257120