[mlir][FuncToLLVM] Add option for emitting opaque pointers

Part of https://discourse.llvm.org/t/rfc-switching-the-llvm-dialect-and-dialect-lowerings-to-opaque-pointers/68179

FuncToLLVM contains some logic working with Memrefs and their lowerings and in the process creating pointer types, loads and allocas. This patch ports the code of these to be compatible with opaque pointers and adds a pass option to enable the use of opaque pointers within the pass.

For the migration effort, the tests have been rewritten to use opaque pointers with dedicated test files for typed pointer support

Differential Revision: https://reviews.llvm.org/D143608