Add return_value_identifier function

[majakusber]

This function was removed in #4616, I would like to bring it back - it makes return_value_symbol more abstract (no need to work with the RETURN_VALUE_SUFFIX string directly), there is at least one place where creating the whole symbol is not necessary and last but not least, the function was quite widely used in test-gen (so in order to update the cbmc pointer, this needs to be resolved).

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[antlechner]

💡 Looks like we no longer need this namespace.