Instead of erroring on actions inside comments, use existing escaping
pipeline to quash the output of actions inside comments.

If a template maintainer uses a comment to disable template code:

  {{if .}}Hello, {{.}}!{{end}}

->

  <!--{{if true}}Hello, {{.}}!{{end}}-->

will result in

  <!--Hello, !-->

regardless of the value of {{.}}.

In a later CL, comment elision will result in the entire commented-out
section being dropped from the template output.

Any side-effects in pipelines, such as panics, will still be realized.

R=nigeltao
CC=golang-dev
https://golang.org/cl/5078041