* Mark some in-class defined member functions as explicitly inline/constexpr, to avoid missing external symbols when using fmt module with shared build due to modules not defaulting to implicit inline.
* Switch constexpr to inline for context::arg(string_view).
NOTE: Looks as if basic_format_args::get(string_view) could probably be made constexpr instead, but sticking with minimal change approach.
* Work around apparent non-conformance of older MSVC compilers.
* Switch format_int::str() from constexpr to inline to satisfy libstdc++ std::string constexpr limitations.
* Replace usages of macros for constexpr/inline with keywords.
* Fix for locations requiring C++14 constexpr.
* Further minor constexpr tweaks.
* Apply clang format
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