go/ast: fix BasicLit.End position for raw strings containing \r

go/ast: fix BasicLit.End position for raw strings containing \r

This CL causes the parser to record in a new field, BasicLit.EndPos,
the actual end position of each literal token, and to use it in
BasicLit.End. Previously, the End was computed heuristically as
Pos + len(Value). This heuristic is incorrect for a multiline
raw string literal on Windows, since the scanner normalizes
\r\n to \n.

Unfortunately the actual end position is not returned by the
Scanner.Scan method, so the scanner and parser conspire
using a global variable in the go/internal/scannerhook
package to communicate.

+ test, api change, relnote

Fixes #76031

Change-Id: I57c18a44e85f7403d470ba23d41dcdcc5a9432c2
Reviewed-on: https://go-review.googlesource.com/c/go/+/720060
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>