Use a less fragile method for defaulting

When doing top-level defaulting, in TcSimplify.applyDefaultingRules, we
were temporarily making a unification variable equal to the default type
(Integer, say, or Float), as a 'given', and trying to solve. But this
relied on the unification variable being untouchable, which seems
complicated.  It's much simpler just to generate a new set of
constraints to solve, using newWantedEvVarNC in disambigGroup.

(I tripped over an ASSERT failure, and this solved it in a robust way.)

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