c-ts-mode: Don't assume comment-start-skip is set

This patch makes it easier to use the existing C and C++
language support in other languages.  Without this patch, if the
outer mode sets some specific and more accurate
comment-start-skip value, or perhaps leaves it unset,
indentation will break.

* lisp/progmodes/c-ts-common.el (c-ts-common--comment-start-skip):
Declare to the value c-ts-common-comment-setup used to set as
comment-start-skip.
(c-ts-common-comment-setup): Use it, rather than hardcoding.
(c-ts-common-comment-2nd-line-anchor): Use it, rather than
comment-start-skip.  This makes it easier to reuse C/++
indentation rules in other TS modes for embedding C/++ segments
in other languages.

Copyright-paperwork-exempt: yes

lisp/progmodes/c-ts-common.el

@@ -90,7 +90,7 @@ non-whitespace characters of the current line."
   (save-excursion
     (forward-line -1)
     (back-to-indentation)
-    (when (looking-at comment-start-skip)
+    (when (looking-at c-ts-common--comment-start-skip)
       (goto-char (match-end 0))
       (if (looking-at (rx (* (or " " "\t")) eol))
           ;; Only /* at the first line.
@@ -274,6 +274,12 @@ This function should be called at BOL.  Used by
    ;; `adaptive-fill-first-line-regexp' decide.
    (t nil)))
 
+(defvar c-ts-common--comment-start-skip
+  (rx (or (seq "/" (+ "/"))
+          (seq "/" (+ "*")))
+      (* (syntax whitespace)))
+  "`comment-start-skip' value for C and C++ tree-sitter modes.")
+
 (defun c-ts-common-comment-setup ()
   "Set up local variables for C-like comment.
 
@@ -291,9 +297,7 @@ Set up:
  - `comment-multi-line'"
   (setq-local comment-start "// ")
   (setq-local comment-end "")
-  (setq-local comment-start-skip (rx (or (seq "/" (+ "/"))
-                                         (seq "/" (+ "*")))
-                                     (* (syntax whitespace))))
+  (setq-local comment-start-skip c-ts-common--comment-start-skip)
   (setq-local comment-end-skip
               (rx (* (syntax whitespace))
                   (group (or (syntax comment-end)