[libc-commits] [libc] [libc] start fix readability-identifier-naming lints (PR #83342)

[Nick Desaulniers via libc-commits]

nickdesaulniers wrote:

Otherwise, if we don't update the clang-tidy rules (and instead have constexpr functions be lowercase, the current number of unique lints here is 11.

So by that measure, I think that it makes sense to:
1. codify that we use lower_case for constexpr functions.
2. fix the 11 violations

Rather then codify it as upper case and fix the 170 resulting violations.

https://github.com/llvm/llvm-project/pull/83342