<xiosbase>: Remove out-of-class definitions of static constexpr data members

[frederick-vs-ja]

Fixes #3379.

It seems that MSVC and Clang (seemly only in clang-cl mode, though) backport the changes of static constexpr data members in P0386R2 to C++14 mode, which should be a conforming extension IIRC (Godbolt example for MSVC). So out-of-class definitions are seemly needless in MSVC STL, even in C++14 mode.

[StephanTLavavej]

I verified that the exports of msvcp140_oss.dll and msvcp140d_oss.dll are unaffected. Both /permissive- and (the infamous, the accursed, the intentionally-no-longer-tested) /Za are fine with this.

Unless we encounter problems in the internal build (e.g. bizarre /clr or /clr:pure interactions), I think this is good. The earlier <random> changes in #3006 were fine, and as I mentioned there, this is how integral_constant::value behaves.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for fixing these compiler warnings and making the codebase less verbose! 🛠️ ⚠️ 😸