Improve ClassTag handling to avoid invalid ClassTag generation and inference failure #16492
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`typeSymbol` returns NoSymbol on an intersection type, so our previous check wasn't good enough. See scala#1730 for background.
The previous commit ensures that `ClassTag[Int & Nothing]` doesn't compile anymore so this isn't a soundness issue anymore, but since intersections can arise out of type inference, it's worth adding a special-case here for convenience. In particular, this fixes scala#16328 (comment)
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The first commit generalizes the fix for #1730, the second commit improves inference in a common situation to generate a valid ClassTag rather than emitting an error, fixing the example from #16328 (comment).