cmd/compile/internal/types2: add debugging support for delayed actions

Add a simple mechanism to provide formatted descriptions for
delayed actions. The comment strings are printed when tracing
is enabled and the delayed action is executed. This results
in more easily decipherable tracing output.

Requires debug mode in order to minimize the overhead during normal
execution. Use the mechanism in a few places to show typical use.

Also cleaned up a few unrelated comments.

Change-Id: Ic273c380c3963341500396ec62b694d143c25de2
Reviewed-on: https://go-review.googlesource.com/c/go/+/355871


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