[onert] Use c++17 features

[hseok-oh]

We are using c++14 build option. By previous task #4932, we can test c++17 features on CI

• Ubuntu: 18.04, gcc-8 toolchain
• Android: android-29 platform, ndk 20, clang 8.0.7 toolchain
• Tizen: gcc-9 toolchain

gcc-8 support c++17 feature without exprimental

Will we change build option to use c++17 ?

https://en.cppreference.com/w/cpp/compiler_support#cpp17

[chunseoklee]

Will we change build option to use c++17 ?

1. tizen's toolchain is gcc-9
2. But, tizen have not suggest to use c++17 officially(AFIAK. let me know if wrong). I think tizen does not guarantee anything for c++17.

[chunseoklee]

Via offline discussion with tizen's toolchain guy, in his personal opinion, there will be no problem with c++17. But it is not official comment :)

[hseok-oh]

Pending issue: x86-64 target is reverted to ubuntu 16.04. So we cannot use gcc-8 toolchain.

[glistening]

I also hope we can enable c++17. However, I prefer c++14 at this moment.

• c++17 is not officially supported from Tizen.
• c++17 is the default from GCC 11. See https://gcc.gnu.org/projects/cxx-status.html#cxx17.
• Tensorflow 2.3 uses GCC 7.3.1 according to tested build configuration. See https://www.tensorflow.org/install/source#cpu.
• Building tensorflow with c++17 had caused some errors. It seems c++14 is officially tested configuration for tensorflow

[chunseoklee]

Let's discuss this later when at least TIZEN officially suggests to use c++17 feature.