clang-fromat-ignore

[Patrick Venture]

I haven't been able to find a good answer on this, but from reading
the man page, there's no built-in mechanism for having clang-format
skip some files.  I wanted to check on this before fixing the script
that calls clang-format in our CI.  I've added a submodule to a
project, and that project doesn't follow our style -- so ignoring the
folder lock, stock, is the right approach.

Patrick