C++/ObjC++: switch to gnu++17 as the default standard

Clang's default C++ standard is now `gnu++17` instead of `gnu++14`:
https://discourse.llvm.org/t/c-objc-switch-to-gnu-17-as-the-default-standard/64360

* CUDA/HIP are unchanged: C++14 from D103221.
* Sony PS4/PS5 are unchanged: https://discourse.llvm.org/t/c-objc-switch-to-gnu-17-as-the-default-standard/64360/6
* lit feature `default-std-cxx` is added to keep CLANG_DEFAULT_STD_CXX=xxx tests working.
  Whether the cmake variable should be retained is disccused in D133375.

Depends on D131464

Reviewed By: #clang-language-wg, aaron.ballman

Differential Revision: https://reviews.llvm.org/D131465