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

[Nikita Popov via llvm-commits]

https://github.com/nikic approved this pull request.

LGTM, but description / title still need an update as this doesn't use enum class.

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