Use const reference in loops if possible.

Removes the `SPIRVAttributes.h` header from `GPU/Transforms/Passes.h`

This commit implements gpu::TargetAttrInterface for SPIR-V target
attribute. The plan is to use this to enable GPU compilation pipeline
for OpenCL kernels later.

The changes do not impact Vulkan shaders using milr-vulkan-runner.
New GPU Dialect transform pass spirv-attach-target is implemented for
attaching attribute from CLI.

gpu-module-to-binary pass now works with GPU module that has SPIR-V
module with OpenCL kernel functions inside.