bug#33973: Multi-line C++ raw string literal font lock is broken

[Li Feng <li.feng@snap.com>]

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Write a multi-line raw string literal in c++-mode, and font
lock doesn't show the correct syntax highlighting. Sample code:

const char* s1 = R"foo(
Hello
World
)foo";

const char* s2 = R"(
Hello
World
)";

In GNU Emacs 27.0.50 (build 1, x86_64-apple-darwin17.7.0, NS appkit-1561.60
Version 10.13.6 (Build 17G65))
 of 2019-01-04 built on C02WP42JHTD6
Repository revision: fe4922cbf01fa9161ffa142fc70972098d48186f
Repository branch: master
Windowing system distributor 'Apple', version 10.3.1561
System Description:  Mac OS X 10.13.6

Recent messages:
Mark set
next-line: End of buffer [15 times]
Mark set
next-line: End of buffer [14 times]
Saving file /Users/lifeng/Desktop/test.cpp...
Wrote /Users/lifeng/Desktop/test.cpp
Mark set [6 times]
Undo!
Making completion list...
user-error: End of history; no default available [2 times]

Configured using:
 'configure --with-modules'

Configured features:
RSVG GLIB NOTIFY KQUEUE ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS NS
MODULES THREADS JSON LCMS2 GMP

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

Major mode: C++//l

Minor modes in effect:
  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
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t
  abbrev-mode: t

Load-path shadows:
None found.

Features:
(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 time-date mm-decode mm-bodies mm-encode mail-parse
rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045
ietf-drums mm-util mail-prsvr mail-utils thingatpt 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 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 threads kqueue cocoa ns lcms2 multi-tty make-network-process
emacs)

Memory information:
((conses 16 227619 13680)
 (symbols 48 22362 1)
 (strings 32 35100 1976)
 (string-bytes 1 1069317)
 (vectors 16 37950)
 (vector-slots 8 757597 18070)
 (floats 8 47 152)
 (intervals 56 520 0)
 (buffers 992 14))

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