Extend object_descriptor_exprt::build and use it

[tautschnig]

Move the implementation from the unit test to object_descriptor_exprt to have
all users of object_descriptor_exprt benefit from it.

• 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).
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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: bb511db).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/98325399

[thk123]

This doesn't handle the case of

**&&foo

or

&*foo // foo is a pointer

but I suppose these cases don't happen in reality?

[thk123]

Not blocking - but now would be a great time to add some unit tests for this utility which would serve both as testing but also documentation for how it can be used.

[smowton]

The first case doesn't make sense (the address of something's address isn't a sensible concept), but &*foo is sane

[smowton]

Maybe briefly comment why this isn't a task for the simplifier?

[tautschnig]

This is something the simplifier should cover, but this piece of code should be usable irrespective of whether an expression has been simplified or not.

[tautschnig]

Test and handling of the &*foo case added.

[allredj]

🚫
This PR failed Diffblue compatibility checks (cbmc commit: 83af955).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/98479178
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.