This was vistigial from the old llvm interpreter, where we it made a bit more
sense.  Now it's just weird, and caused the tiering logic to be spread into
weird places.  It was also the source of some bugs, since when we would deopt
there's not really any relevant CompiledFunction that represents the deopt
frame (this is why type speculation had to be temporarily disabled).

So instead, make it so that the interpreter (and by extension, the baseline
jit) work on the CLFunction directly, since the CompiledFunction didn't hold
any relevant information anyway.  This require a whole bunch of refactoring and
API changes.

This commit doesn't actually change any of the tiering decisions (I think), but
should just make them clearer; the actual behavioral changes will be done in
upcoming commits.