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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> > 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.
> 
> @arsenm thoughts? We need to have a prefix to avoid conflicting with reserved names, but would you be ok with not having an `enum class`/`namespace` for the impls?

Yes, I don't think the enum class is necessary. RTLIB was preexisting, we could pull everything out if it if desired. 

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