check_clang_tidy.py now passes `-format-style=none` to clang_tidy

Summary:
If the test does not specify a formatting style, force "none"; otherwise
autodetection logic can discover a ".clang-tidy" file that is not
related to the test.

Reviewers: alexfh

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61739

llvm-svn: 360358