[clang-tidy] readability-avoid-const-params-in-decls template instantiation bugfix

Summary: Bugfix for 30398.  Don't warn for template instantiations

Reviewers: aaron.ballman, hokein, alexfh

Subscribers: omtcyfz, cfe-commits

Differential Revision: https://reviews.llvm.org/D24652

llvm-svn: 283873