Allow line comments ending with escaped NL to be continued to the next line.

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'.