Cleanup unwind table emission code a bit.

This change removes the `tidyLandingPads` function, which previously
had a few responsibilities:

1. Dealing with the deletion of an invoke, after MachineFunction lowering.
2. Dealing with the deletion of a landing pad BB, after MachineFunction lowering.
3. Cleaning up the type-id list generated by `MachineFunction::addLandingPad`.

Case 3 has been fixed in the generator, and the others are now handled
during table emission.

This change also removes `MachineFunction`'s `addCatchTypeInfo`,
`addFilterTypeInfo`, and `addCleanup` helper fns, as they had a single
caller, and being outlined didn't make it simpler.

Finally, as calling `tidyLandingPads` was effectively the only thing
`DwarfCFIExceptionBase` did, that class has been eliminated.