go/ast: record start and end of file in File.File{Start,End}

go/ast: record start and end of file in File.File{Start,End}

This change causes the parser to record the positions of the first
and last character in the file in new ast.File fields FileStart
and FileEnd.

The behavior of the existing Pos() and End() methods,
which record the span of declarations, must remain unchanged
for compatibility.

Fixes golang/go#53202

Change-Id: I250b19e69f41e3590292c3fe6dea1943ec98f629
Reviewed-on: https://go-review.googlesource.com/c/go/+/427955
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Alan Donovan <adonovan@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>