[NativeProcessLinux] Fix assertion failure when killing a process

Linux sometimes sends us a PTRACE_EVENT_EXIT when an inferior process gets a SIGKILL. This can be
confusing, since normally we don't expect any events when the inferior is stopped. This commit
adds code to handle this situation (resume the thread and let it exit normally) and avoid an
assertion failure in ResumeThread().

llvm-svn: 246539