[mlir][LLVM] Revert bareptr calling convention handling as an argument materialization.

Type conversion and argument materialization are context-free: there is no available information on which op / branch is currently being converted.
As a consequence, bare ptr convention cannot be handled as an argument materialization: it would apply irrespectively of the parent op.
This doesn't typecheck in the case of non-funcOp and we would see cases where a memref descriptor would be inserted in place of the pointer in another memref descriptor.

For now the proper behavior is to revert to a specific BarePtrFunc implementation and drop the blanket argument materialization logic.

This reverts the relevant piece of the conversion to LLVM to what it was before https://reviews.llvm.org/D105880 and adds a relevant test and documentation to avoid the mistake by whomever attempts this again in the future.

Reviewed By: arpith-jacob

Differential Revision: https://reviews.llvm.org/D106495