Use a source generator to generate boilerplate for TransformerBase

[PathogenDavid]

~~This generator should ideally be written so that non-standard declarations can be added easily.~~ Custom declaration and type reference support is handled through special interfaces. So we don't need to worry about supporting them. (Although we could automagically implement these interfaces.)

[PathogenDavid]

We should also generate GetEnumerator on declarations which have translated declarations as children.