Refactor ToIrListener id generation

[shonfeder]

This is a small refactor supporting #1082

Add a helper function to generate location IDs and register them into
the source map. These must always go together, so adding the two into
the same function saves a number of lines and makes the code safer since
it ensures we won't miss a needed addition to the map.

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• [-] Tests added for any new code
• [-] Documentation added for any new functionality
• [-] Entries added to the respective CHANGELOG.md for any new functionality
• [-] Feature table on README.md updated for any listed functionality

[thpani]

Looks good!

Is the side-effect obvious enough to not reflect it in the name (e.g. getIdAndUpdateSourceMap() vs getId())?

[shonfeder]

Thanks for the review!

Is the side-effect obvious enough to not reflect it in the name

Given that the method is private and commented, I feel it is nice and convenient to phrase it is I did here. The map updates is really an implementation detail happening behind the scenes, and there's no reason for the adder for a visitor method to think about it.

I could encapsulate the id gen and map update in a class, but seems a bit overkill here.