[NativeProcessLinux] Support inferiors which change their process group

Summary:
Previously, we wait()ed for events from the inferiors process group. This is resulted in a
failure if the inferior changed its process group in the middle of execution. To avoid this, I
pass -1 to the wait() call. The flag __WNOTHREAD makes sure we don't actually wait for events
from any process, but only the processes(threads) which are our children (or traced by us). Since
this happens on the monitor thread, which is dedicated to monitoring a single inferior, we will
be getting events only from this inferior.

Test Plan: All tests pass on linux. I have added a test to check the new functionality.

Reviewers: chaoren, ovyalov

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D10061

llvm-svn: 238405