goto_rw: store function name alongside instruction reference [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. goto_rw does not yet make use
of this, but will need to once dereferencing is updated to require a function
name.

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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 2cf0109).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/97892874