Implemented clang-tidy configurability via .clang-tidy files.

Summary:
This adds a support for the .clang-tidy file reading using
FileOptionsProvider, -dump-config option, and changes tests to not depend on
default checks set.

Reviewers: klimek, bkramer, djasper

Reviewed By: djasper

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D5186