Use tcEqType in GHC.Core.Unify.uVar

  Because uVar used eqType instead of tcEqType, it was possible
  to accumulate a substitution that unified Type and Constraint.

  For example, a call to `tc_unify_tys` with arguments

    tys1 = [ k, k ]
    tys2 = [ Type, Constraint ]

  would first add `k = Type` to the substitution. That's fine, but then
  the second call to `uVar` would claim that the substitution also
  unifies `k` with `Constraint`. This could then be used to cause
  trouble, as per #20521.

  Fixes #20521

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