Simple conflict-resolution for JSON symbol tables

[xbauch]

Link JSON symbol tables via linking(old_symtab, new_symtab, message_handler).

Instead of trying to merge them and throwing exception for every symbol
conflict.

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• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
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[smowton]

You might want to look at whether the linker used for combining multiple C symbol tables suits your needs

[xbauch]

@smowton Is this what you had in mind?

[smowton]

If it suits your needs, yes

[tautschnig]

Is there a chance to have this tested in some way? Though what I'm more worried about: the linker currently applies conflict resolution rules that seem acceptable with C code. Other languages might be more strict about those conflicts.

[xbauch]

@tautschnig I've added a regression test here: a88a188. And then found out that there is a discrepancy between the condition in remove_returns and its validation check. The last commit (16e2a14) unifies them but I can also make it into a separate PR if that would be more appropriate.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 16e2a14).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/111724200