Flattening of ID_nondet_symbol for unbounded arrays [blocks: #3619]

[tautschnig]

symbol_exprt and nondet_symbol_exprt are to be treated the same when in comes to
the back-end: they both introduce an uninterpreted, nullary function. Note the
difference at the level of symbolic execution/programs, where the value of
symbols may change (while the value of nondet_symbol_exprts cannot).

This is factored out from #3619 to make it easier to pinpoint what exactly causes trouble with TG. The commit is exactly the same as the first one from #3619, which has been approved already.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 915bb93).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/99326800

[tautschnig]

Marking do-not-merge until explicitly approved by @romainbrenguier or anyone else via a TG test run.

[smowton]

Approving as part of an approved PR

[allredj]

🚫
This PR failed Diffblue compatibility checks (cbmc commit: e2a7181).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/99415339
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.

[tautschnig]

@romainbrenguier confirmed that this passes TG tests.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 909a29e).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/99575614