[mlir] provide same APIs as existing LLVMType in the new LLVM type modeling

These are intended to smoothen the transition and may be removed in the future
in favor of more MLIR-compatible APIs. They intentionally have the same
semantics as the existing functions, which must remain stable until the
transition is complete.

Depends On D85019

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D85020