unreachable_instructions now passes down function identifier [blocks: #3126]

[tautschnig]

We are working towards removing the "function" field from
goto_programt::instructionst::instructiont, and thus need to pass the identifier
of the function whenever it actually is required.

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• 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.
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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: c8b463a).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/97868058

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: b6f6ebe).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/98712313