LLVM: properly define a type for `!llvm.struct` instead of UnregisteredAttr

[GZGavinZhao]

From #592. Right now we parse a !llvm.struct into an opaque UnregisteredAttr. Ideally this should be parsed into something analogous to LLVM.ArrayType.

Reference definition from upstream MLIR: https://github.com/llvm/llvm-project/blob/367498977c36bdb723d5298f6637c8a9a5c74d77/mlir/include/mlir/Dialect/LLVMIR/LLVMTypes.td#L150