[CUDA][HIP] Use the same default language std as C++

Currently CUDA/HIP defines their own language standards in
LanguageStandards.def but they are redundant. They are the same as stdc++14.
The fact that CUDA/HIP uses c++* in option -std= indicates that they have the
same language standards as C++. The CUDA/HIP specific language features are
conveyed through language options, not language standards features. It makes
sense to let CUDA/HIP uses the same default language standard as C++.

Reviewed by: Siu Chi Chan, Artem Belevich

Differential Revision: https://reviews.llvm.org/D155539

Fixes: SWDEV-407685