compiler: Check for invalid UTF8 in Go comments.
compiler: Check for invalid UTF8 in Go comments.

Fixes golang/go#11527.

Change-Id: I94fd08ae67393c3422858bc6d1d1311d590c842f
Reviewed-on: https://go-review.googlesource.com/13905
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend