[mlir][llvm] Reintroduce string based attribute setting.

Reintroduce string based attribute setting in the
translation from LLVM dialect to LLVM IR. The TypeSwitch
based implementation introduced in
https://reviews.llvm.org/D143654  does not work for
intrinsics that set the requiresAccessGroup or
requiresAliasScope flag.

Reviewed By: hgreving

Differential Revision: https://reviews.llvm.org/D143936