Create SSA-level 2 names for individual fields

[tautschnig]

During symex, a declaration introduces non-deterministic values for the
declared symbol by creating a fresh L2 name without assigning any value
to it. With field-sensitive SSA, the same must be done for each field
(just like we already did the inverse for objects going out of scope via
a DEAD instruction).

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[smowton]

This is surely either-or -- if we're doing this we should not create the non-canonical symbol l1_identifier. So I'd say remove the if(fields != ssa) and the increase_generation call directly above it.

[tautschnig]

Done.

[allredj]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: fbd06ba).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/108692125
Status will be re-evaluated on next push.
Common spurious failures include: 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); compatibility was already broken by an earlier merge.

[allredj]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: 24f4674).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/108697316
Status will be re-evaluated on next push.
Common spurious failures include: 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); compatibility was already broken by an earlier merge.