[mlir] Prepare convert-gpu-to-spirv for OpenCL support (#69941)

This includes a couple of changes to pass behavior for OpenCL kernels.
Vulkan shaders are not impacted by the changes.

1. SPIR-V module is placed inside GPU module. This change is required for
gpu-module-to-binary to work correctly as it expects kernel function to
be inside the GPU module.
2. A dummy func.func with same kernel name as gpu.func is created. GPU
compilation pipeline defers lowering of gpu launch kernel op. Since
spirv.func is not directly tied to gpu launch kernel, a dummy func.func
is required to avoid legalization issues.
3. Use correct mapping when mapping MemRef memory space to SPIR-V
storage class for OpenCL kernels.