Fix comment detection on open parens

Fix comment detection on open parens

Characters having both open paren syntax and comment start syntax were
being detected as open parens even when they should have been part a
comment starter (Bug#24870).

* src/syntax.c (in_2char_comment_start): New function, extracted from
`scan_sexps_forward'.
(scan_sexps_forward): Add check for a 2-char comment starter before the
loop.  Inside the loop, do that check after incrementing the 'from'
character index.  Move the single char comment syntax cases into the
switch instead of special casing them before.
* test/src/syntax-tests.el (parse-partial-sexp-paren-comments):
(parse-partial-sexp-continue-over-comment-marker): New tests.