<locale>: Hide some non-Standard functions of locale::id

[frederick-vs-ja]

The non-Standard constructor and conversion function are likely to change the behavior of overload resolution, so perhaps they should be eliminated. But they're currently appearing in the DLL interface and thus can't be removed until vNext.

This PR attempts to make them only exist when building the STL. The newly added Standard default constructor and _Get_index function are templates to avoid alternating DLL interface. (I'm still unsure whether this is sufficient or necessary.)

[StephanTLavavej]

The newly added Standard default constructor and _Get_index function are templates to avoid alternating DLL interface. (I'm still unsure whether this is sufficient or necessary.)

When a class is separately compiled, as class _CRTIMP2_PURE_IMPORT id is, then any ordinary member functions will be separately compiled and dllexported.

As always, you can check by running dumpbin /exports on the DLL and comparing before-and-after.

Edit: This change is special because of the default argument (review comment incoming).

[StephanTLavavej]

Thanks! 🐈 I pushed a commit to fix bincompat, plus a couple of minor commits. This is moderately risky but I'm willing to roll the dice.

[StephanTLavavej]

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

[StephanTLavavej]

Looks like we avoided rolling snake eyes!

⛔ 🐍 🎲 🎲