[msvc] fix warning about non-inline variable

[russelltg]

With MSVC 17.4.4, I'm getting a warning originating from fmt headers:

fmt\core.h(2909,15): warning C5260: the constant variable 'fmt::v9::detail::invalid_arg_index' has internal linkage in an included header file context, but external linkage in imported header unit context; consider declaring it 'inline' as well if it will be shared across translation units, or 'static' to express intent to use it local to this translation unit
fmt\core.h(2909,15): message : to simplify migration, consider the temporary use of /Wv:18 flag with the version of the compiler with which you used to build without warnings
fmt\core.h(413,58): warning C5260: the constant variable 'fmt::v9::detail::micro' has internal linkage in an included header file context, but external linkage in imported header unit context; consider declaring it 'inline' as well if it will be shared across translation units, or 'static' to express intent to use it local to this translation unit
fmt\core.h(413,58): message : to simplify migration, consider the temporary use of /Wv:18 flag with the version of the compiler with which you used to build without warnings
fmt\format.h(639,20): warning C5260: the constant variable 'fmt::v9::detail::invalid_code_point' has internal linkage in an included header file context, but external linkage in imported header unit context; consider declaring it 'inline' as well if it will be shared across translation units, or 'static' to express intent to use it local to this translation unit
fmt\format.h(639,20): message : to simplify migration, consider the temporary use of /Wv:18 flag with the version of the compiler with which you used to build without warnings

It seems to me inline is the proper linkage specifier here, as the definition won't vary from compilation unit to compilation unit.

However, since it's only a C++17 feature, I've only enabled it when the feature is detected.

[russelltg]

I tried to run clang-format, but it touched lines that I didn't touch. What is the suggested clang-format version so I can make sure I'm formatting the code correctly?

[vitaut]

Thanks for the PR! Merged with a minor tweak in 6e6eb63. Most of the code is formatted with clang-format 12.