[mlir] translate types between MLIR LLVM dialect and LLVM IR

With new LLVM dialect type modeling, the dialect types no longer wrap LLVM IR
types. Therefore, they need to be translated to and from LLVM IR during export
and import. Introduce the relevant functionality for translating types. It is
currently exercised by an ad-hoc type translation roundtripping test that will
be subsumed by the actual translation test when the type system transition is
complete.

Depends On D84339

Reviewed By: herhut

Differential Revision: https://reviews.llvm.org/D85019