Give the MCStreamer class hierarchy LLVM RTTI facilities for use with
isa<> and dyn_cast<>. In several places, code is already hacking around
the absence of this, and there seem to be several interfaces that might
be lifted and/or devirtualized using this.

This change was based on a discussion with Jim Grosbach about how best
to handle testing for specific MCStreamer subclasses. He said that this
was the correct end state, and everything else was too hacky so
I decided to just make it so.

No functionality should be changed here, this is just threading the kind
through all the constructors and setting up the classof overloads.