Make sure the "synchronous breakpoint callbacks" get called before the thread...

Make sure the "synchronous breakpoint callbacks" get called before the thread plan logic gets invoked, and if they
ask to continue that should short-circuit the thread plans for that thread.  Also add a bit more explanation for
how this machinery is supposed to work.  
Also pass eExecutionPolicyOnlyWhenNeeded, not eExecutionPolicyAlways when evaluating the expression for breakpoint
conditions.

llvm-svn: 155236