clang-format.el: Make clang-format work in indirect buffers.

In an indirect buffer, buffer-file-name is nil, so check the base buffer
instead.  This works fine in direct buffers where buffer-base-buffer returns
nil.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D120408

Author: phst

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

@@ -147,7 +147,7 @@ uses the function `buffer-file-name'."
     (setq style clang-format-style))
 
   (unless assume-file-name
-    (setq assume-file-name buffer-file-name))
+    (setq assume-file-name (buffer-file-name (buffer-base-buffer))))
 
   (let ((file-start (clang-format--bufferpos-to-filepos start 'approximate
                                                         'utf-8-unix))