go/ast: add Range token.Pos to RangeStmt

go/ast: add Range token.Pos to RangeStmt

For #50429

Change-Id: Idb027244f901d9f482c894b5b979a054d0f07de5
Reviewed-on: https://go-review.googlesource.com/c/go/+/426091
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>