go/types, types2: prevent unification from recursing endlessly

This is a stop gap solution to avoid panics due to stack overflow
during type unification. While this doesn't address the underlying
issues (for which we are still investigating the correct approach),
it prevents a panic during compilation and reports a (possibly not
quite correct) error message.

If the programs are correct in the first place, manually providing
the desired type arguments is a viable work-around, resulting in
code that will continue to work even when the issues here are fixed
satisfactorily.

For #48619.
For #48656.

Change-Id: I13bb14552b38b4170b5a1b820e3172d88ff656ec
Reviewed-on: https://go-review.googlesource.com/c/go/+/377954
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

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