[ClangFormat] Editor integrations inherit default style from clang-format binary

Summary:
This allows downstream customizations to the default style to work without
needing to also modify the editor integrations.

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D49719

llvm-svn: 360996