Allow underscore in block return type even if the type can't be computed

[asterite]

Fixes #10932

Alternative to #10929

[straight-shoota]

I'm a bit hesitant about merging such a compiler change last minute. But I suppose we might just postpone the release for another day or so to see if it creates any smoke 🤷
And it is probably the right thing to do, so we might as well do it now.

I can confirm this fixes the issue discovered in crinja (#10905 (comment)), so this fix is also an alternative to #10928.

[asterite]

This fix makes the compiler not check a type where it previously did. Types are eventually checked when needed, anyway. So I'm 100% confident that this won't break anything.

[beta-ziliani]

Either case we might break things (adding annotations, removing or changing them), so I say let's go with this better approach.

[straight-shoota]

test-ecosystem is green: https://github.com/crystal-lang/test-ecosystem/actions/runs/1026623854 👌