Investigate why the Block TypeAssigner avoids types but not terms

[smarter]

See e7e643c and the follow-up discussion at #2945 (comment). Ever TypeAssigner needs to be changed to avoid everything or we need to document why there's a special case.

[dwijnand]

@smarter Is this still true and need doing? I'm thinking of looking at it myself with Seth.

[smarter]

No idea. I forgot the context here, but long term I think we should see if we can get rid of TypeOps#avoid in favor of LevelAvoidMap as I outlined in https://github.com/lampepfl/dotty/blob/1292246c9646db12391cb7e42ce951722ac7fd03/compiler/src/dotty/tools/dotc/core/TypeOps.scala#L494-L498 which came out of #14026

[odersky]

@smarter Feel free to close if you think this is addressed now.