go/types, types2: use | rather than ∪ when printing term lists

go/types, types2: use | rather than ∪ when printing term lists

With this change, the termlist String() function prints termlists
in the usual Go notation and thus we can use it in error reporting.
Preparation for fixing #40350.

For #40350.

Change-Id: Ia28318841305de234a71af3146ce0c59f5e601a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/410894
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>