dialyzer: Fix another pattern literal bug

dialyzer_typesig:traverse/3 would perform an unsafe optimisation when
given a cons pattern that contained a map and could be folded into a
literal with cerl:fold_literal/1. In this case, when traversing the map
a type variable would be generated, but this variable would be dropped
by the erl_types:t_cons/2 constructor by in turn calling t_sup(),
producing the overapproximation any(). However, in this particular case,
dialyzer_typesig:traverse/3 is not allowed to overapproximate, since its
result is used in an EQ-constraint.

Although erl_types:t_tuple/1 does not overapproximate like t_cons/2,
which makes the bug unlikely to affect tuples too, the fix was
nevertheless applied defensively to the case of tuples as well.

Also, fix a bug where dialyzer_utils:refold_pattern/1 would generate
syntax nodes with two {label, _} attributes.

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