This PR adds lemmas for iterated conversions between finite types,
starting with something of type `Nat`/`Int`/`Fin`/`BitVec` and going
through `IntX`.
Makes the LLVM triple of the current platform available to Lean code
towards a solution for #2754.
Defaults to the empty string if the compiler is not clang, which can
introduce some divergence between CI and local builds but should not be
noticeable in most cases and is not really possible to avoid.
