go/*: use go/types.Func.Signature and go/ast.Preorder

go/*: use go/types.Func.Signature and go/ast.Preorder

In the few obvious candidates that I found after a bit of grepping.

Change-Id: I36af79c46d29e4422bce1f43bbbac9db7de2001a
Reviewed-on: https://go-review.googlesource.com/c/go/+/650656
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
Commit-Queue: Alan Donovan <adonovan@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>