[Rua]

• Depends on transpile: Move handling of ToUnion and NullToPointer casts to convert_cast #2050

This reduces duplicated code in convert_indecrement_operator, and in convert_floating_literal itself by letting make_cast handle the f128 case. It also moves some of the "does this type fit" handling into convert_floating_literal. That means that an implicit convert_cast and convert_expr on a float are completely equivalent from here: convert_cast will always propagate the target type to convert_expr via expected_type_id, where convert_floating_literal will handle any incompatibilities.