Handle \-newline inside a C++ // comment.
(skip_if_group): Likewise.
(handle_directive): Also handle C++ // comment after #.  Point
ip->bufp after the comment start before calling
skip_to_end_of_comment.
(validate_else): Handle \-newline inside a C++ // comment.
(skip_to_end_of_comment, macarg1, discard_comments): Likewise.

From-SVN: r7028