[CUDA][HIP] Add pre-defined macro `__CLANG_RDC__`

nvcc defines __CUDACC_RDC__ for both host and
device compilation when -rdc=true is specified
(https://docs.nvidia.com/cuda/cuda-compiler-driver-nvcc/index.html#nvcc-identification-macro)

This patch defines __CLANG_RDC__ when -fgpu-rdc
is specified for CUDA/HIP.

Reviewed by: Artem Belevich

Differential Revision: https://reviews.llvm.org/D114812