swift234

IRGen: Implement resilient enum case numbering

Recent changes added support for resiliently-sized enums, and
enums resilient to changes in implementation strategy.

This patch adds resilient case numbering, fixing the problem
where adding new payload cases would break existing code by
changing the numbering of no-payload cases.

The problem is that internally, enum cases are numbered with payload
cases coming first, followed by no-payload cases. While each list
is itself in declaration order, with new additions coming at the
end, we need to partition it to give us a fast runtime test for
"is this a payload or no-payload case index."

The resilient numbering strategy used here is that the getEnumTag
and destructiveInjectEnumTag value witness functions now take a
tag index in the range [-ElementsWithPayload..ElementsWithNoPayload-1].

Payload elements are numbered in *reverse* declaration order, so
adding new payload cases yields decreasing tag indices, and adding
new no-payload cases yields increasing tag indices, allowing use
sites to be resilient.

This adds the adjustment between 'fragile' and 'resilient' tag
indices in a somewhat unsatisfying manner, because the calculation
could be pushed down further into EnumImplStrategy, simplifying
both the IRGen code and the generated IR. I'll clean this up later.

In the meantime, clean up some other stuff in GenEnum.cpp, mostly
abstracting code that walks cases.