Add automatic formatting on save for C++ using ClangFormat

This can be enabled by setting both `c-c++-enable-clang-format-on-save` and
`c-c++-enable-clang-support` to `t`.

layers/+lang/c-c++/README.org

@@ -84,6 +84,14 @@ particular mode or all modes in your =dotspacemacs/user-config= (within your
       (define-key c++-mode-map [tab] 'clang-format-buffer))
 #+END_SRC
 
+To enable automatic buffer formatting on save, set the variable
+=c-c++-enable-clang-format-on-save= to =t=:
+
+#+BEGIN_SRC emacs-lisp
+  (setq-default dotspacemacs-configuration-layers '(
+    (c-c++ :variables c-c++-enable-clang-format-on-save t)))
+#+END_SRC
+
 *** Company-clang and flycheck
 This layer adds some fancy improvements to =company-clang=.
 It includes a hook to load a projects =.clang_complete= file, which is

layers/+lang/c-c++/config.el

@@ -14,6 +14,10 @@
 (defvar c-c++-enable-clang-support nil
   "If non nil Clang related packages and configuration are enabled.")
 
+(defvar c-c++-enable-clang-format-on-save nil
+  "If non-nil, automatically format code with ClangFormat on
+  save. Clang support has to be enabled for this to work.")
+
 (spacemacs|define-jump-handlers c++-mode)
 (spacemacs|define-jump-handlers c-mode)
 

layers/+lang/c-c++/funcs.el

@@ -9,6 +9,10 @@
 ;;
 ;;; License: GPLv3
 
+(defun clang-format/format-on-save ()
+  "Format buffers with ClangFormat when they get saved."
+  (add-hook 'before-save-hook 'clang-format-buffer nil t))
+
 (defun company-mode/more-than-prefix-guesser ()
   (c-c++/load-clang-args)
   (company-clang-guess-prefix))

layers/+lang/c-c++/packages.el

@@ -61,7 +61,11 @@
 
 (defun c-c++/init-clang-format ()
   (use-package clang-format
-    :if c-c++-enable-clang-support))
+    :if c-c++-enable-clang-support
+    :init
+    (when c-c++-enable-clang-format-on-save
+      (spacemacs/add-to-hooks 'clang-format/format-on-save
+                              '(c-mode-hook c++-mode-hook)))))
 
 (defun c-c++/init-cmake-mode ()
   (use-package cmake-mode