[clang-tools-extra] [clang-tidy] Fix readability-identifier-naming FP with DefaultCase on function templates (PR #189788)

[Zeyi Xu via cfe-commits]

zeyi2 wrote:

> Do variable templates or class templates have the same problem?

After some experimenting, yes they do. Thanks!

Fixing `TypeAliasTemplateDecl` is a bit tricky, so I probably need more time to work on this.

https://github.com/llvm/llvm-project/pull/189788