[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:

The PR title and description are out of date with the implementation.

TBH I liked your original Impl_ prefix more than lci_. `RTLIB::Impl_` is more obvious than `RTLIB::lci_` if you're not very familiar with this code and know that `lci` is short for `LibCallImpl`.

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