[mlir][llvmir] Add support for llvm.signext/zeroext function attributes.

This change-set adds basic support for llvm.signext and llvm.zeroext
attributes, and makes sure that the attributes are translated to LLVM IR
when attached to arguments.

This is needed for https://github.com/llvm/llvm-project/issues/58579

Differential Revision: https://reviews.llvm.org/D137048