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

[Daniel Paoliello via llvm-commits]

dpaoliello wrote:

Ok, executive decision, I'm going with `RTLIB::impl_`, it makes it clear for the code outside of RuntimeLibcalls what these enum values are. Within RuntimeLibcalls, there is a lot of "impl" next to "impl", but I don't think it's a bad thing: it makes it clear that the two are related (e.g., a function and this enum).

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