Output clang-tidy errors in a consistent order.

Summary: Sort errors by path, file offset and message.

Reviewers: djasper, klimek

Reviewed By: djasper

CC: cfe-commits

Differential Revision: http://reviews.llvm.org/D3314

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@205759 91177308-0d34-0410-b5e6-96231b3b80d8