clang-format: Add a fallback style to Emacs mode

Summary:
This allows one to enable `clang-format-buffer` on file save and avoid
reformatting files that are outside of any project with .clang-format style.

Reviewers: djasper, klimek, sammccall, owenpan, mitchell-stellar, MyDeveloperDay

Reviewed By: MyDeveloperDay

Subscribers: cfe-commits

Patch By: dottedmag

Tags: #clang, #clang-format

Differential Revision: https://reviews.llvm.org/D69752