Add __attribute__((const)) to grid related functions declarations
Add __attribute__((const)) to grid related functions declarations

Make declarations consistent with https://github.com/RadeonOpenCompute/ROCm-Device-Libs/blob/amd-stg-open/ockl/src/workitem.cl
Without the attribute these functions don't have "readnone" LLVM IR attribute. Without it some optimizations fails, e.g. Loop Invariant Code Motion doesn't hoist these calls out of a loop.
GPUOpen-ProfessionalCompute-Tools/HIP,GPUOpen-ProfessionalCompute-Tools/HIP,GPUOpen-ProfessionalCompute-Tools/HIP,GPUOpen-ProfessionalCompute-Tools/HIP,GPUOpen-ProfessionalCompute-Tools/HIP