clang-format: Add a fallback style to Emacs mode

[dottedmag]

Summary

This allows one to enable clang-format-buffer on file save and avoid
reformatting files that are outside of any project with .clang-format style.

[MyDeveloperDay]

I'm not an Emacs user, but this LGTM as much as I can read the rest of the file.

[dottedmag]

I am not a committer, so please commit.

clang/tools/clang-format/clang-format.el

 By default clang-format will load the style configuration from
 a file named .clang-format located in one of the parent directories
 of the buffer."
   :group 'clang-format
   :type '(choice (string) (const nil))
   :safe #'stringp)
 (make-variable-buffer-local 'clang-format-style)
 
+(defcustom clang-format-fallback-style "none"
+  "Fallback style to pass to clang-format.
+
+This style will be used if clang-format-style is set to \"file\"
+and no .clang-format is found in the directory of the buffer or
+one of parent directories. Set to \"none\" to disable formatting
+in such buffers."
+  :group 'clang-format
+  :type 'string
+  :safe #'stringp)
+(make-variable-buffer-local 'clang-format-fallback-style)
+
 (defun clang-format--extract (xml-node)
   "Extract replacements and cursor information from XML-NODE."
   (unless (and (listp xml-node) (eq (xml-node-name xml-node) 'replacements))
     (error "Expected <replacements> node"))
   (let ((nodes (xml-node-children xml-node))
         (incomplete-format (xml-get-attribute xml-node 'incomplete_format))
         replacements
         cursor)
(unwind-protect
                                ;; Guard against a nil assume-file-name.
                                ;; If the clang-format option -assume-filename
                                ;; is given a blank string it will crash as per
                                ;; the following bug report
                                ;; https://bugs.llvm.org/show_bug.cgi?id=34667
                                ,@(and assume-file-name
                                       (list "-assume-filename" assume-file-name))
                                ,@(and style (list "-style" style))
+                               "-fallback-style" ,clang-format-fallback-style
                                "-offset" ,(number-to-string file-start)
                                "-length" ,(number-to-string (- file-end file-start))
                                "-cursor" ,(number-to-string cursor))))
               (stderr (with-temp-buffer
                         (unless (zerop (cadr (insert-file-contents temp-file)))
                           (insert ": "))
                         (buffer-substring-no-properties
                          (point-min) (line-end-position)))))