Recreate old less safe for loop behavior #18262
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Our behavior changed with #17404 to disallow doing
for..inon unions containing type variables (which we have allowed in the past, despite its lack of extra safety). With this change we allow it once more, by introducing a propagating flag which tracks the presence of a type variable. Technically, only unions and intersections needed to be special-cased (as anywhere else the flag is propagated will be caught by theNonPrimitiveassignability check), but I feel the flag is much cleaner than specializingisTypeAssignableToKind.Fixes #18203.