Add TestConcurrentEvents test for LLDB's handling of inferior threads

- Test verifies LLDB's handling of inferiors with threads that: hit breakpoints,
  modfiy variables that have watchpoints set, generate user signals, and crash.
- Add a few "stress tests" (with ~100 threads) -- run these with "-l" dotest.py
  flag.
- Fix stop_reason_to_str helper in lldbutil to handle eStopReasonThreadExited.
- Add sort_stopped_threads helper to lldbutil to separate thread lists based
  on stop reason.

Logged llvm.org/pr16566 and llvm.org/pr16567 for bugs exposed.

llvm-svn: 185889