[clang-tidy] misc-unused-parameters: don't comment out parameter name for C code

Summary: The fixit `int square(int /*num*/)` yields `error: parameter name omitted` for C code. Enable it only for C++ code.

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

Subscribers: xazax.hun, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63088

llvm-svn: 364106