go/ast: add File.GoVersion

go/ast: add File.GoVersion

For #57001, compilers and others tools will need to understand that
a different Go version can be used in different files in a program,
according to the //go:build lines in those files.

This CL adds a GoVersion string field to ast.File, to allow exposing this
per-file Go version information.

For #59033.

Change-Id: I3931ea86c237983d152964f48dce498bcb1f06aa
Reviewed-on: https://go-review.googlesource.com/c/go/+/476276
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>