Add a "force_kill" arg to Process::Destroy(). This is needed after

the changes in r233255/r233258.  Normally if lldb attaches to
a running process, when we call Process::Destroy, we want to detach
from the process.  If lldb launched the process itself, ::Destroy
should kill it.

However, if we attach to a process and the driver calls SBProcess::Kill()
(which calls Destroy), we need to kill it even if we didn't launch it
originally.

The force_kill param allows for the SBProcess::Kill method to force the
behavior of Destroy.

<rdar://problem/20424439> 

llvm-svn: 235158