Preprocessor: Don't treat a line containing multiple comments as a blank line.

After this patch, 'a', 'b', and 'c' are all treated as members of the
same list in the following wikitext:

*a
 <!--x-->
*b
 <!--x--> <!--y-->
*c

The old comment-removal rule was "trim a comment which is both
preceded and followed by a newline (ignoring spaces)".  This only works
if there is a single comment on the line, and was often surprising
to users.  The new rule allows any number of whitespace-separated
comments on the line.

Bug: 41756
Change-Id: I6030086226e1eeece59643c29dbb4361668b4bd6