Allow line comments ending with escaped NL to be continued to the next line.
Use this in C, C++, and Objective C Modes. Fixes bug#22246
* src/syntax.c (comment-end-can-be-escaped): New buffer local variable.
(forw-comment, back-comment): On encountering an end of comment character,
test whether it is escaped when `comment-end-can-be-escaped' is non-nil.
* doc/lispref/syntax.texi (Control Parsing): Describe
`comment-end-can-be-escaped'.
* etc/NEWS (Lisp Changes): Describe `comment-end-can-be-escaped'.
* lisp/progmodes/cc-langs.el: New c-lang-setvar `comment-end-can-be-escaped'.