[CUDA][HIP] Change default lang std to c++14

Currently clang and nvcc use c++14 as default std for C++.
gcc 11 even uses c++17 as default std for C++. However,
clang uses c++98 as default std for CUDA/HIP.

As c++14 has been well adopted and became default for
clang, it seems reasonable to use c++14 as default std
for CUDA/HIP.

Reviewed by: Artem Belevich

Differential Revision: https://reviews.llvm.org/D103221