CC Mode (some languages): handle string lines ending in \\

CC Mode (some languages): handle string lines ending in \\

In C, C++, Objective C, and Pike modes, regard \\ in a string
at EOL as a backslash followed by an escaped newline.  In the
other languages, this remains regarded as an escaped backslash
followed by an invalid string terminator.

* lisp/progmodes/cc-defs.el (c-is-escaped, c-will-be-escaped):
Amend to observe the changed notion of escaped newlines.

* lisp/progmodes/cc-langs.el (c-string-escaped-newlines):
Unused, removed.
(c-escaped-newline-takes-precedence): New lang const and var.
(c-string-innards-re-alist): Amend, using the above new lang
var.