<random>: Remove non-Standard inheritance from piecewise distributions

[frederick-vs-ja]

By changing the discrete_distribution<size_t> base class subobjects to leading member subobjects. This doesn't change layout of the piecewise_{constant,linear}_distribution objects as said in the comments. Also do this for their param_type.

This results in API breakages but is probably safe for ABI.

Fixes #1600.

[StephanTLavavej]

Thank you, this is great!! 😻 I pushed minor changes.

Importantly, I manually compared old vs. new for debug/release x64/x86, verifying not only the sizeof and alignof but that the layouts were effectively identical after the base class surgery. This is the same technique that I used for my previous <random> layout overhauls.

#include <print>
#include <random>
#include <string_view>
using namespace std;

template <typename T>
void test(const string_view name) {
    println("sizeof({}): {}", name, sizeof(T));
    println("alignof({}): {}", name, alignof(T));
}

int main() {
#ifdef _DEBUG
    println("Debug; {}-bit; _MSVC_STL_UPDATE: {}", sizeof(void*) * 8, _MSVC_STL_UPDATE);
#else
    println("Release; {}-bit; _MSVC_STL_UPDATE: {}", sizeof(void*) * 8, _MSVC_STL_UPDATE);
#endif
    test<piecewise_constant_distribution<float>>("piecewise_constant_distribution<float>");
    test<piecewise_constant_distribution<double>>("piecewise_constant_distribution<double>");
    test<piecewise_constant_distribution<float>::param_type>("piecewise_constant_distribution<float>::param_type");
    test<piecewise_constant_distribution<double>::param_type>("piecewise_constant_distribution<double>::param_type");
    test<piecewise_linear_distribution<float>>("piecewise_linear_distribution<float>");
    test<piecewise_linear_distribution<double>>("piecewise_linear_distribution<double>");
    test<piecewise_linear_distribution<float>::param_type>("piecewise_linear_distribution<float>::param_type");
    test<piecewise_linear_distribution<double>::param_type>("piecewise_linear_distribution<double>::param_type");
}

struct Meow1 : piecewise_constant_distribution<float> {};
struct Meow2 : piecewise_constant_distribution<double> {};
struct Meow3 : piecewise_constant_distribution<float>::param_type {};
struct Meow4 : piecewise_constant_distribution<double>::param_type {};
struct Meow5 : piecewise_linear_distribution<float> {};
struct Meow6 : piecewise_linear_distribution<double> {};
struct Meow7 : piecewise_linear_distribution<float>::param_type {};
struct Meow8 : piecewise_linear_distribution<double>::param_type {};

cl /EHsc /nologo /W4 /std:c++latest /MTd /Od /d1reportSingleClassLayoutMeow meow.cpp && meow
cl /EHsc /nologo /W4 /std:c++latest /MT /Od /d1reportSingleClassLayoutMeow meow.cpp && meow

[StephanTLavavej]

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