Fix filling in c-ts-mode (bug#72116)

Fix filling in c-ts-mode (bug#72116)

The previous fix introduced a regression in the case when there's only a
single line in the block comment.  In that case we don't want to add a
start at the second line:

/* foo foo foo */ should => /* foo foo
                               foo */

rather than   /* foo foo
               * foo */

This commit fixes that.

* lisp/progmodes/c-ts-common.el:
(c-ts-common--fill-block-comment): Don't mask the /*.
(c-ts-common--adaptive-fill-prefix): New function.
(c-ts-common-comment-setup): Don't set adaptive-regexp, change
adaptive-fill-first-line-regexp to work with the new
adaptive-fill-function.
* test/lisp/progmodes/c-ts-mode-resources/filling.erts: New tests

(cherry picked from commit 74bb1e5897f4532fbdefddada28258a8d3d5c95f)