Code generator should handle flattened ARM ID's better

[matthchr]

Describe the current behavior
Today, there is a special flattenedPropertyHandler handler for properties that have been flattened, and a referencePropertyHandler for properties that are references. Unfortunately, if a property is both flattened and a reference, it ends up taking the flattenedPropertyHandler path which assumes that the property names on both sides line up. For reference properties they don't as one is xReference and one is xId (usually).

Describe the improvement
Today we have special one-off handling in the flattenedPropertyHandler to deal with this case, where it checks for the presence of astmodel.ARMReferenceTag and honors that rename instead. Instead, we should consider making a standard way to rename properties so that we don't need to duplicate this logic in two handlers. Additionally if more causes for renames come along, checking each specific one will be difficult whereas if we have a standard one in theory it will be easier.

We also need to add tests for these interaction cases as they can be quite tricky. Unfortunately right now flattening isn't easily testable with golden files.

[theunrepentantgeek]

We still need to do this.

[theunrepentantgeek]

It's possible we no longer need this now #2323 has been merged. We need to check.

[matthchr]

No we still need to do this I think. Not high priority though.

[matthchr]

No change from above.