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.

As we saw in #20521 (closed), this can be used to cause trouble.