C++17 support?

[Lei YU]

Now we are using GCC 7.3 and it has full C++17 support, although it is
[experimental][1]

Is it suitable for a product to use C++17 instead of C++14 in repos?

The reason I am asking is because phosphor-time-manager got a build error when
enabled with LinkTimeOptimization, related to an undefined reference to a
`static constexpr char*`, and it could be avoided if I change to use C++17 to
compile, because constexpr implies inline in C++17 and it fixes the issue.
So I would like to use C++17 in this repo, which then does not require any
code change to enable LinkTimeOptimization.

Be noted that although we are using C++14, some repos are using C++17 features
in std::experimental namespace already, e.g. std::experimental ::filesystem.

[1]: https://gcc.gnu.org/projects/cxx-status.html#cxx17