This is some groundwork for filtering the language Exception

breakpoints, for instance on the class of the thrown object.

This change doesn't actually make that work, the part where we
extract the thrown object type from the throw site isn't done yet.

This provides a general programmatic "precondition" that you can add
to breakpoints to give them the ability to do filtering on the LLDB
side before we pass the stop on to the user-provided conditions & 
callbacks.

llvm-svn: 235538