Trigger assertions to detect UB on self-emplace and self-generic-assignment

[saki7]

Self-emplace and self-generic-assignment on non-valueless instance ALWAYS leads to UB, because the standard specifies emplace to destruct the contained object before emplacing the new value:

[variant.mod]
Effects: Destroys the currently contained value if valueless_by_exception() is false. Then direct-non-list-initializes...

This does apply also to generic assignment operator as the standard says it delegates to emplace on certain branches:

[variant.assign]
Otherwise, if is_nothrow_constructible_v<Tj, T> || !is_nothrow_move_constructible_v<Tj> is true, equivalent to emplace<j>(std​::​forward<T>(t)).

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Our version (rvariant version) of the specification ([rvariant.mod]) allows creating temporary variable on recursive_wrapper, making it bulletproof to self-emplace. However, there's no point of conditionally omitting the assertion on that branch, because doing so would only make the codebase error-prone and has no benefit for end users.

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Also fixes the wrong semantics where our implementation of generic assignment operator was not conforming to the standard, creating temporary object in non-permitted branch.

[cppwarningnotifier]

Environment				C++23	C++26
ubuntu24.04	14	g++-14	Debug	✅success	✅success
			Release	✅success	✅success
	21	clang++-21	Debug	✅success	✅success
			Release	✅success	✅success
	22	clang++-22	Debug	✅success	✅success
			Release	✅success	✅success
windows2022	v143	cl	Debug	✅success	✅success
			Release	✅success	✅success