clang-format: Add -assume-filename option for editor integrations.

With -style=file, clang-format now starts to search for a .clang-format
file starting at the file given with -assume-filename if it reads from
stdin. Otherwise, it would start searching from the current directory,
which is not helpful for editor integrations.

Also changed vim, emacs and sublime integrations to actually make use of
this flag.

This fixes llvm.org/PR17072.