lang: Update the Build signature to return a type

This returns the type with the arg names we'll actually use. This is
helpful so we can pass values to the right places. We have named edges
so you can actually see what's going on.

Co-authored-by: Samuel Gélineau <gelisam@gmail.com>

lang/ast/structs.go

@@ -6938,9 +6938,12 @@ func (obj *ExprFunc) SetType(typ *types.Type) error {
 	if obj.Function != nil {
 		polyFn, ok := obj.function.(interfaces.PolyFunc) // is it statically polymorphic?
 		if ok {
-			if err := polyFn.Build(typ); err != nil {
+			newTyp, err := polyFn.Build(typ)
+			if err != nil {
 				return errwrap.Wrapf(err, "could not build expr func")
 			}
+			// Cmp doesn't compare arg names.
+			typ = newTyp // check it's compatible down below...
 		}
 	}