[clang-tidy] RenamerClangTidy now correctly renames `using namespace` decls

Summary: Fixes [[ https://bugs.llvm.org/show_bug.cgi?id=45039 | readability-identifier-naming doesn't rename using namespace correctly. ]]

Reviewers: aaron.ballman, gribozavr2, JonasToth, hokein, alexfh

Reviewed By: aaron.ballman

Subscribers: xazax.hun, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D75220