[clang-tidy] Add a checker that flags unnamed parameters.

Summary:
We still allow the escape hatch foo(int /*x*/) and also suggest this
in a fixit. This is more powerful than the corresponding cpplint.py check
it also flags functions with multiple arguments as naming all arguments is
recommended by the google style guide.

Reviewers: alexfh, djasper

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D4518