[CIR][AMDGPU] Add AMDGPU-specific function attributes for HIP/OpenCL kernels

[skc7]

This patch adds support for AMDGPU-specific function attributes for HIP/OpenCL kernels

• Added setTargetAttributes for AMDGPUTargetCIRGenInfo to set kernel attributes
• Added generic string attribute handler in amendFunction to translate string-valued extra function attributes to LLVM function attributes

Follows OGCG AMDGPU implementation from "clang/lib/CodeGen/Targets/AMDGPU.cpp".

[skc7]

All the attributes lowered in this PR are amdgpu llvm backend specific and I think CIR need not have a separate attribute for them at HIP level, (which prevents leakage of LLVM specific things into CIR).