go/types, types2: re-enable panic if unification stack-overflows
With all the unification/type-inference fixes in place now, we
should not see stack overflows anymore. Re-enable the panic if
we do overflow, so we can address those issues should they arise.
Fixes #51377.
Change-Id: Ied64435ea5936811504cb30bda1126c7d85980f8
Reviewed-on: https://go-review.googlesource.com/c/go/+/392755
Trust: Robert Griesemer <gri@golang.org> Reviewed-by: Robert Findley <rfindley@google.com>