[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