[clang-tidy] Bring order to check registration.

Summary:
Register readability checks in a separate module. Renamed the checks
and test file names accordingly.

Reviewers: djasper, klimek

Reviewed By: klimek

Subscribers: curdeius, cfe-commits

Differential Revision: http://reviews.llvm.org/D5936

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@220631 91177308-0d34-0410-b5e6-96231b3b80d8