[MLIR] Support global_memref and get_global_memref in standard -> LLVM conversion.

• Convert global_memref to LLVM::GlobalOp.
• Convert get_global_memref to a memref descriptor with a pointer to the first element of the global stashed in it.
• Extend unit test and a mlir-cpu-runner test to validate the generated LLVM IR.

Differential Revision: https://reviews.llvm.org/D90803