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

NOKEYCHECK=True
GitOrigin-RevId: 2f906683ed14668ceed1f85922789abffd8429fd
1 file changed