[clang-tools-extra] [clang-tidy] Add more constexpr options to `readability-identifier-naming` (PR #162160)

[Ewan Green via cfe-commits]

ewancg wrote:

It is logically questionable but every C++ resource I know about refers to constants defined with `constexpr` as "constexpr variables". Here is a long list of references to it in the standard: https://github.com/search?q=repo%3Acplusplus%2Fdraft%20constexpr%20variable

https://github.com/llvm/llvm-project/pull/162160