[mlir][gpu] Introduce `host_shared` flag to `gpu.alloc`

Motivation: we have lowering pipeline based on upstream gpu and spirv dialects and and we are using host shared gpu memory to transfer data between host and device.
Add `host_shared` flag to `gpu.alloc` to distinguish between shared and device-only gpu memory allocations.

Differential Revision: https://reviews.llvm.org/D133533