<tuple>, <variant>: Make error messages from get-by-type functions clearer

[A. Jiang (frederick-vs-ja)]

Fixes #4575 by aborting recursive instantiation using if constexpr.
Also reuses _Meta_find_unique_index for overloads for tuple.

[Stephan T. Lavavej (StephanTLavavej)]

Thanks! 😻 Your new static_asserts are beautiful:

D:\GitHub\STL\out\x64>cl /EHsc /nologo /W4 /std:c++latest /MTd /Od /c meow.cpp
meow.cpp
D:\GitHub\STL\out\x64\out\inc\tuple(957): error C2338: static_assert failed: 'get<T>(tuple<Types...>&) requires T to occur exactly once in Types. (N4971 [tuple.elem]/5)'
D:\GitHub\STL\out\x64\out\inc\tuple(957): note: the template instantiation context (the oldest one first) is
meow.cpp(5): note: see reference to function template instantiation '_Ty &std::get<float,int,long,double>(std::tuple<int,long,double> &) noexcept' being compiled
        with
        [
            _Ty=float
        ]

D:\GitHub\STL\out\x64>clang-cl /EHsc /nologo /W4 /std:c++latest /MTd /Od /c meow.cpp
In file included from meow.cpp:1:
D:\GitHub\STL\out\x64\out\inc\tuple(957,23): error: static assertion failed due to requirement '_Always_false<float>':
      get<T>(tuple<Types...>&) requires T to occur exactly once in Types. (N4971 [tuple.elem]/5)
  957 |         static_assert(_Always_false<_Ty>,
      |                       ^~~~~~~~~~~~~~~~~~
meow.cpp(5,12): note: in instantiation of function template specialization 'std::get<float, int, long, double>'
      requested here
    5 |     (void) get<float>(t);
      |            ^
1 error generated.

I pushed a simplification/throughput improvement and a Standard citation fix.

[Stephan T. Lavavej (StephanTLavavej)]

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

[Stephan T. Lavavej (StephanTLavavej)]

Thanks again for this massive diagnostic improvement! 🪄 🚀 😻