[CUDA][HIP] Do not diagnose use of _Float16

r352221 caused regressions in CUDA/HIP since device function may use _Float16 whereas host does not support it.
In this case host compilation should not diagnose usage of _Float16 in device functions or variables.

For now just do not diagnose _Float16 for CUDA/HIP. In the future we should have more precise check.

Differential Revision: https://reviews.llvm.org/D57369

llvm-svn: 352488