go/parser: set File{Start,End} correctly in all cases

go/parser: set File{Start,End} correctly in all cases

...even when the file is empty or lacks a valid package decl.

+ test

Fixes #70162

Change-Id: Idf33998911475fe8cdfaa4786ac3ba1745f54963
Reviewed-on: https://go-review.googlesource.com/c/go/+/624655
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>