Standardize traverse union behavior in `TypeChecker`s.

[PieterCK]

Some TypeCheckers match_annotation implementation doesn't traverse unions, while others do.

Comment from @ntBre:

... I feel like we should probably either always traverse unions or never traverse unions and apply that decision to all of our TypeCheckers rather than keep a mix of them.

[ntBre]

Thanks for opening the issue!

I don't feel too strongly here either way, and I haven't vetted all the uses of all the TypeCheckers to be sure if this would work, but at a high level it makes sense to me to be consistent. I can also see an argument either way, to always do this or never do it, but lean towards always traversing unions myself. I'm interested to hear other thoughts!