go/types: add commentMap and test

go/types: add commentMap and test

This adds the (adjusted) syntax.CommentMap function and corresponding
test to the types_test package so that we can use it for collecting
ERROR comments in the next CL.

For #51006.

Change-Id: I63ce96e7394c28c02d5a292250586cc49c1f6e19
Reviewed-on: https://go-review.googlesource.com/c/go/+/456125
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>