Fix assume-filename handling in clang-format.el

Summary:
When 'buffer-file-name' is nil 'call-process-region' returned a segmentation fault error.

This was a problem when using clang-format-buffer on an orgmode source code editing buffer.

I fixed this problem by excluding the '-assume-filename' argument when 'buffer-file-name' is nil.

To make it a bit more flexible I also added an optional argument, 'assume-file-name', to specify an assume-filename that overrides 'buffer-file-name'.

Reviewers: klimek, djasper, phst, phi

Reviewed By: phst, phi

Subscribers: phi, jholewinski, mgorny, javed.absar, eraman, cfe-commits

Differential Revision: https://reviews.llvm.org/D37903

llvm-svn: 319621