bug#29339: 27.0.50; CC-mode doesn't know about .mm and .M suffixes for Objective-C++

[Philipp <p.stephani2@gmail.com>]

GCC uses .mm or .M for Objective-C++,
cf. https://gcc.gnu.org/onlinedocs/gcc-7.2.0/gcc/Overall-Options.html.
However, neither `auto-mode-alist' nor `cc-other-file-alist' contain
these file extensions.


In GNU Emacs 27.0.50 (build 17, x86_64-pc-linux-gnu, GTK+ Version 3.10.8)
 of 2017-11-17 built on localhost
Repository revision: 345c7d9c7b84c0c03ef1d0f32e9e0f39029be44a
Windowing system distributor 'The X.Org Foundation', version 11.0.11803000
System Description:	Ubuntu 14.04.5 LTS

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Configured using:
 'configure --enable-gtk-deprecation-warnings --with-modules
 --without-pop --with-mailutils --enable-checking
 --enable-check-lisp-object-type --enable-gcc-warnings 'CFLAGS=-ggdb3
 -O0''

Configured features:
XPM JPEG TIFF GIF PNG SOUND GSETTINGS NOTIFY GNUTLS FREETYPE XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 MODULES

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

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  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

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny seq byte-opt gv
bytecomp byte-compile cconv cl-loaddefs cl-lib dired dired-loaddefs
format-spec rfc822 mml easymenu 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 elec-pair time-date
mule-util tooltip eldoc electric uniquify ediff-hook vc-hooks
lisp-float-type mwheel term/x-win x-win term/common-win x-dnd 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 inotify dynamic-setting
system-font-setting font-render-setting move-toolbar gtk x-toolkit x
multi-tty make-network-process emacs)

Memory information:
((conses 16 94754 12856)
 (symbols 48 20187 1)
 (miscs 40 45 145)
 (strings 32 28237 1585)
 (string-bytes 1 737043)
 (vectors 16 14050)
 (vector-slots 8 498529 13298)
 (floats 8 53 64)
 (intervals 56 214 0)
 (buffers 992 12)
 (heap 1024 24710 963))

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