Update the unwind_diagnose script to work correctly with both

lldb-179 version numberings and the new lldb-300 version numberings.
Remove the pretense that someone might run this from the command
line; this is only used from within a live lldb debug session.  Fix
the loading so it can be loaded via "script import lldb.macosx" or
the script can be loaded individually like "command script import
unwind_diagnose.py"

llvm-svn: 180085