Beautifier: Add -assume-filename to clang-format

Otherwise -style=file will be unusable, since clang-format needs to know
original file path in order to resolve .clang-format file location.

Change-Id: I0581cbcdf5770eed98179fe37d5517f648d7b826
Reviewed-by: Lorenz Haas <lykurg@gmail.com>
Reviewed-by: Przemyslaw Gorszkowski <pgorszkowski@gmail.com>
Reviewed-by: David Schulz <david.schulz@digia.com>