Previously we can only use unwind information at the beginning of a block
since we otherwise have no label to apply it to. This, of course, isn't
nearly expressive enough to properly encode unwinding for general
programs. For this reason, we want to be able to include unwind
information at arbitrary points within a block.

This is a first step in this direction, ensuring that all unwind nodes
We accomplish this by making `CmmUnwind` carry a label, which the NCG
will emit into the block and which the Dwarf generator can refer to in
the unwind tables it produces.