go/types, types2: add a const to control recursion panics in unification

Add a panicAtUnificationDepthLimit const to replace the use of the debug
const to control whether to panic when the unification recursion depth
is reached. Our tests should pass when debug==true.

Change-Id: I58847f49d66010bd4ca01c0408ec10acac95cba6
Reviewed-on: https://go-review.googlesource.com/c/go/+/382534


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>