Enable FMT_STRING() use with types other than string literals

[scramsby]

This enables using FMT_STRING() with raw character arrays, like:

constexpr char msg[5] = {'h', 'e', 'l', 'l', 'o'};
FMT_STRING(msg);

...and other string view types like:

constexpr std::string_view msg{"hello"};
FMT_STRING(msg);

Also fixes some constexpr evaluation build errors when building with C++17 with arg_ref internal class.

I agree that my contributions are licensed under the {fmt} license, and agree to future changes to the licensing.

[vitaut]

Thanks for the PR. Could you explain the motivation for this change?

[scramsby]

Our coding standards use string_view and equivalent types for defining shared string constants as they are generally more type safe. This enables us to continue to use such types for use in our logging system which uses fmt. The change in the terminating null character detection is mostly just because I noticed it while working in the same area, we don't have concrete need for it currently.

[vitaut]

Makes sense. Please add unit tests to test/format-test.cc and address inline comments.

[scramsby]

Added unit test when building with C++17.

[vitaut]

Two more comments, otherwise looks good.

[vitaut]

There are some build failures in CI.

[scramsby]

Finally got the changes trimmed down to what is needed and fixed the last few niggling platform-specific build errors, so this should be good to go absent further feedback.

[vitaut]

Merged with minor tweaks in 664dd88. Thanks!