bug#34791: 27.0.50; Raw C++ strings are not font-locked correctly

[Ivan Andrus]

Raw C++ strings used to be font-locked correctly, and now they are not.
I bisected and found bb591f139f0602af292c772f974dcc14dabb1deb to be the
first bad commit.

An example as

auto rsi = R"JSON(
                {
                 "calendarType": "GREGORIAN",
                 "timezone": 21,
                 "currentTimezoneOffset": 1
                }
               )JSON"_Json;

fails to font-lock correctly.

-Ivan

In GNU Emacs 27.0.50 (build 2, x86_64-apple-darwin18.2.0, NS appkit-1671.20 
Version 10.14.3 (Build 18D109))
of 2019-03-08 built on iandrus-macOS
Repository revision: bb591f139f0602af292c772f974dcc14dabb1deb
System Description:  Mac OS X 10.14.3

Recent messages:
Auto-saving...done
Auto-saving...done
next-line: End of buffer [8 times]
report-emacs-bug-hook: M-x report-emacs-bug was canceled, please read *Bug 
Help* buffer
Auto-saving...done
Sending...
message-send: No methods specified to send by
Sending...
message-send: No methods specified to send by
Undo!

Configured using:
'configure --with-ns --with-gnutls=no
PKG_CONFIG_PATH=/opt/X11/lib/pkgconfig'

Configured features:
RSVG NOTIFY ACL LIBXML2 ZLIB TOOLKIT_SCROLL_BARS NS THREADS

Important settings:
 value of $LANG: en_US.UTF-8
 locale-coding-system: utf-8-unix

Major mode: Message

Minor modes in effect:
 mml-mode: t
 diff-auto-refine-mode: t
 tooltip-mode: t
 global-eldoc-mode: t
 electric-indent-mode: t
 mouse-wheel-mode: t
 tool-bar-mode: t
 menu-bar-mode: t
 file-name-shadow-mode: t
 global-font-lock-mode: t
 font-lock-mode: t
 auto-composition-mode: t
 auto-encryption-mode: t
 auto-compression-mode: t
 line-number-mode: t
 auto-fill-function: message-do-auto-fill
 transient-mark-mode: t
 abbrev-mode: t

Load-path shadows:
None found.

Features:
(help-mode pp shadow sort mail-extr emacsbug message rmc puny seq
byte-opt gv bytecomp byte-compile cconv dired dired-loaddefs format-spec
rfc822 mml mml-sec password-cache epa derived epg epg-config gnus-util
rmail rmail-loaddefs mm-decode mm-bodies mm-encode mail-parse rfc2231
mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045 ietf-drums
mm-util mail-prsvr mail-utils vc-git diff-mode easy-mmode cc-mode
cc-fonts easymenu cc-guess cc-menus cc-cmds cc-styles cc-align cc-engine
cc-vars cc-defs cl-loaddefs cl-lib term/xterm xterm time-date elec-pair
tooltip eldoc electric uniquify ediff-hook vc-hooks lisp-float-type
mwheel term/ns-win ns-win ucs-normalize mule-util term/common-win
tool-bar dnd fontset image regexp-opt fringe tabulated-list replace
newcomment text-mode elisp-mode lisp-mode prog-mode register page
menu-bar rfn-eshadow isearch timer select scroll-bar mouse jit-lock
font-lock syntax facemenu font-core term/tty-colors frame cl-generic
cham georgian utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao
korean japanese eucjp-ms cp51932 hebrew greek romanian slovak czech
european ethiopic indian cyrillic chinese composite charscript charprop
case-table epa-hook jka-cmpr-hook help simple abbrev obarray minibuffer
cl-preloaded nadvice loaddefs button faces cus-face macroexp files
text-properties overlay sha1 md5 base64 format env code-pages mule
custom widget hashtable-print-readable backquote kqueue cocoa ns
multi-tty make-network-process emacs)

Memory information:
((conses 16 279002 20021)
(symbols 48 23274 1)
(miscs 40 58 403)
(strings 32 36862 2508)
(string-bytes 1 1119931)
(vectors 16 37375)
(vector-slots 8 731781 18434)
(floats 8 55 332)
(intervals 56 13800 180)
(buffers 992 15))