Two tweaks enable this:
1) Updating the raw and data span pointers when Tokenizer.Next is called, even
if an error has occurred. This prevents duplicate data from being returned by
Raw in the common case of an EOF.

2) Treating '</>' as an empty comment token to expose the raw text as a
tokenization event. (This matches the semantics of other non-token events,
e.g., '</ >' is treated as '<!-- -->'.)

Fixes golang/go#7029.

R=golang-codereviews, r, bradfitz
CC=golang-codereviews
https://golang.org/cl/46370043