LLGS local - signal stops inferior in debugger

NativeProcessLinux::MonitorSignal was automatically resuming threads
that stopped due to a signal.  This is inconsistent with the
behavior of lldb and gdb.  This change removes the automatic resume.

Fixes
TestSendSignal.py
TestSignalsAPI.py
if PLATFORM_LINUX_FORCE_LLGS_LOCAL is in the environment vars.

llvm-svn: 227918