Add a new test in api/multiple-debuggers which tries to create 50

Add a new test in api/multiple-debuggers which tries to create 50
debug sessions simultaneously to expose race conditoin/locking
issues.

This directory has an inferior program, testprog.cpp that has a
couple of functions we can put breakpoints on.

It has a driver program, multi-process-driver.cpp, which links
against the LLDB solib and uses the SB APIs.  It creates 50 pthreads,
creates a debugger on all of them, launches a debug session of the
inferior testprog, hits a couple breakpoints, walks the stack,
continues, etc., and then kills the inferior and ends the debug
session.

A pass is if all fifty debug sessions complete successfully
in the alloted time (~60 seconds).

We may need to tweak this one to work correctly on different
platforms/targets but I wanted to get it checked in to start.

llvm-svn: 212671