(c-beginning-of-statement):

(c-beginning-of-statement):
Fixes in sentence movement to properly
handle M-e moving forward into a comment when looking at preceding
whitespace, and M-a moving backward into comment when looking at
following whitespace.

Uncommented the looking-at call in the
sentence-flag clause so that moving by forward-sentence when looking
at the beginning of a comment works again.  A previous log message in
cc-mode.el indicates this was commented out "because
c-beginning-of-statement-1 should do the right thing", but clearly it
doesn't.

I don't know if this breaks something else, because I can't figure out
why it was commented out in the first place.