Move Broadcaster+Listener+Event combo from Core into Utility
These are general purpose "utility" classes, whose functionality is not
debugger-specific in any way. As such, I believe they belong in the
This doesn't break any particular dependency (yet), but it reduces the
number of Core dependencies across the board.
Reviewers: zturner, jingham, teemperor, clayborg
Subscribers: mgorny, lldb-commits
Differential Revision: https://reviews.llvm.org/D55361