go/types, types2: move xlist next to targs in Checker.arguments signature

go/types, types2: move xlist next to targs in Checker.arguments signature

targs and xlist belong together (xlist contains the type expressions for
each of the type arguments).

Also, in builtins.go, rename xlist to alist2 to avoid some confusion.

Preparation for adding more parameters to the Checker.arguments signature.

Change-Id: I960501cfd2b88410ec0d581a6520a4e80fcdc56a
Reviewed-on: https://go-review.googlesource.com/c/go/+/494121
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>