Add a first pass at a "stop hook" mechanism.  This allows you to add commands that get run every time the debugger stops, whether due to a breakpoint, the end of a step, interrupt, etc.  You can also specify in which context you want the stop hook to run, for instance only on a particular thread, or only in a particular shared library, function, file, line range within a file.

Still need to add "in methods of a class" to the specifiers, and the ability to write the stop hooks in the Scripting language as well as in the Command Language.

llvm-svn: 127457