[clang-tidy] misc-unused-parameters - retain old behavior under StrictMode

Summary: This addresses https://bugs.llvm.org/show_bug.cgi?id=37467.

Reviewers: klimek, ilya-biryukov, lebedev.ri, aaron.ballman

Reviewed By: lebedev.ri, aaron.ballman

Subscribers: aaron.ballman, lebedev.ri, xazax.hun, cfe-commits

Differential Revision: https://reviews.llvm.org/D46951

llvm-svn: 335863