readability-identifier-naming needs to differentiate constexpr variables

[HazardyKnusperkeks]

We have GlobalConstant, LocalConstant, StaticConstant, etc. But constexpr just applies to everything.

I want globals, file globals, and static members which are constexpr to have a different style than function local constexpr variables.

[llvmbot]

@llvm/issue-subscribers-clang-tidy