smt2 id_map: Do not use nil_typet when we mean "optional" [blocks: #3800]

[tautschnig]

Use optionalt instead. nil_typet is deprecated.

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[allredj]

🚫
This PR failed Diffblue compatibility checks (cbmc commit: 8033ac4).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/98644406
Status will be re-evaluated on next push.
Please contact @peterschrammel, @thk123, or @allredj for support.

Common spurious failures:

• the cbmc commit has disappeared in the mean time (e.g. in a force-push)
• the author is not in the list of contributors (e.g. first-time contributors).

The incompatibility may have been introduced by an earlier PR. In that case merging this
PR should be avoided unless it fixes the current incompatibility.

[kroening]

I believe it's an invariant that the type is always set -- get_fresh_id could do a .find() instead, and then .emplace could be used to insert.

[tautschnig]

I'll re-architect this one as suggested.

[tautschnig]

@kroening Re-architected in a way that seems overall cleaner to me - but it isn't a part of the code base that I know all that well.

[kroening]

idt::type is now redundant -- that information is always in idt::definition as well. I am mildly uneasy about a NIL expression whose type is used in an important way; I don't have a better idea right now.

[kroening]

I think what we would want here is a variant type; so a cheap substitute is to have both the typet and the exprt, both optional, and exactly one is used.

[kroening]

If this is indeed a case of a variant type, then #3962 may appeal.

[tautschnig]

It seems it only almost is a case of a variant type: it's not quite the case for define-fun. This was a place where my initial patch had introduced a new bug, and also where a bug already existed (and is now fixed), see the included extension of a regression test.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 7792612).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/98740109