This PR converts the `std::variant` summary from Python to C++.
Split from #148554. MSVC's STL and libstdc++ use the same type name for
`std::variant`, thus they need one "dispatcher" function that checks the
type and calls the appropriate summary. For summaries, both need to be
implemented in C++.
This is mostly a 1:1 translation. The main difference is that in C++,
the summary returns `false` if it can't inspect the object properly
(e.g. a member could not be found). In Python, this wasn't possible.
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