Improve Unit ascription escape hatch

[som-snytt]

Fixes #23145

I typoed my branch name.

As in Scala 2, use TreeInfo to deal with the hassle of adapted trees.

The interesting case is this.m which is a lossy adaptation. The attachments must be copied.

[som-snytt]

Forgot to update the check file when line numbers changed. Possibly the check file is useful for checking that the warning "looks good" (carets are correct, types are printed in desired form).