Add .clang-tidy configuration file to provide LLVM-optimized defaults for clang-tidy.

Reviewers: chandlerc, djasper, alexfh

Reviewed By: alexfh

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D5237

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@217366 91177308-0d34-0410-b5e6-96231b3b80d8