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>

src/plugins/beautifier/clangformat/clangformat.cpp

@@ -145,6 +145,7 @@ Command ClangFormat::command(int offset, int length) const
         command.addOption(QLatin1String("-offset=") + QString::number(offset));
         command.addOption(QLatin1String("-length=") + QString::number(length));
     }
+    command.addOption(QLatin1String("-assume-filename=%file"));
 
     return command;
 }