fixups aka the stubs that decref whatever's needed when an exception is thrown

I looked into this because most (75%?) of the refcounting overhead
comes from the cxx fixups.  Previously we would always generate them in
the IRGenerator, regardless of whether they were needed.  Now they are
generated in the refcounter, which knows whether they are needed or not.

Unfortunately it looks like they are usually needed, so the gains here
aren't that great (saves about 10% llvm instructions whereas cxx fixups
in general added about 400% more llvm instructions).

I think this is still a good change because it's also necessary in order to use
Marius's EH stuff.

I think the cost of the fixups is mostly related to the cost of the decrefs
that it adds, so even though most of the refcounting overhead seems to be due to
adding the cxx fixups, reducing general decref overhead might reduce cxx fixup overhead