[CIR][AMDGPU] Add AMDGPU target support for AST to CIR codegen

[skc7]

This PR implements AMDGPU target support for AST->CIR codgen.

• Adds AMDGPUTargetCIRGenInfo with:
  • getOpenCLKernelCallingConv() returning AMDGPUKernel
  • setCUDAKernelCallingConvention() for HIP/CUDA kernel support

With changes in PR:

• OpenCL kernels on AMDGPU get cc(amdgpu_kernel) calling convention
• HIP __global__ kernels get cc(amdgpu_kernel) calling convention

[bcardosolopes]

This is not the right place for callconv lowering, there's a dedicated pass for this, you should teach that instead.

[skc7]

This is not the right place for callconv lowering, there's a dedicated pass for this, you should teach that instead.

@bcardosolopes
Updated PR to add amdgpu_kernel CC enum and removed any CallingConv lowering in TargetInfo module.