<random>: Improvements around static constexpr member variables

[frederick-vs-ja]

This PR proposes the following improvements for some static member variables in <random>:

1. _Swc_01_traits will have all of its static member variables statically initialized.
2. No redeclarations (since C++17, definitions in C++14 and former revisions) of constexpr static member variables will ever exist, even if one attempt to revive tr1 in C++17 (and later modes). They are deprecated in C++17, even though warnings seem to be silenced.
3. (According to the suggestion) Out-of-class definitions will also be removed in C++14 mode.

The first should be reverted if changing from dynamic initialization to static one or removal of odr-using breaks ABI.

[StephanTLavavej]

Looks good, thanks! I hope /clr:pure lets us do this (I am hopeful as the static constexpr data members aren't themselves templated).

[strega-nil-ms]

rest LGTM; not marking request changes since I may get disagreement on the badness of ldexp_one

[StephanTLavavej]

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

[StephanTLavavej]

Thanks for randomly hacking the codebase into a better state! ⌨️ 🖥️ 😻