(font-lock-no-comments): New variable.

(font-lock-no-comments): New variable.
(font-lock-after-change-function): Test it.
(font-lock-fontify-buffer): Test it.
(font-lock-set-defaults): Set it.

(font-lock-mode): Make font-lock-no-comments local.
(font-lock-after-change-function): If we don't call
font-lock-fontify-region, clear out face props instead.

(font-lock-fontify-buffer): Don't give "syntactically"
message if we aren't doing that part.