cmd/compile: fix irgen mis-handling of ... argument when creating closure

When bulding formal arguments of newly created closure, irgen forgets to
set "..." field attribute, causing type mismatched between the closure
function and the ONAME node represents that closure function.

Fixes #49432

Change-Id: Ieddaa64980cdd3d8cea236a5a9de0204ee21ee39
Reviewed-on: https://go-review.googlesource.com/c/go/+/361961


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>