[llvm] [NFC] RuntimeLibcalls: Prefix the impls with 'Impl_' and use an enum class (PR #153850)

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

If you're giving this an `Impl_` prefix, I'm not sure it makes sense to *also* use `enum class`. That double-namespaces all symbols.

https://github.com/llvm/llvm-project/pull/153850