A piecewise's domain condition asks every case's predicate to be defined #1817

A piecewise is defined where the case it takes is, but DomainCondition asks every case's predicate to be defined too, and an ordering of a quantity that is not real never is:

"piecewise(1 provided not i in RR, 2 provided i < 0)".ToEntity().Evaled           // 1
"piecewise(1 provided not i in RR, 2 provided i < 0)".ToEntity().DomainCondition   // False
"piecewise(x provided not x = 0, 3*x^2 provided i - 1 < 0)".ToEntity().DomainCondition   // False

Piecewise.InnerSimplify already passes over a predicate that evaluates to NaN; the domain condition does not, because it is the predicates' disjunction and the domain condition of every child, and c < 0 is defined only where c is real.

It reaches integration through a substitution that carries the domain condition of the answer it found back with it. Rubi 4.3.2.1's 1/(sqrt(a + i a tan(x)) (c + d tan(x))^(3/2)), in u = tan(x), is answered on master with an arm for each sign of a discriminant 8 a^3 c - 8 i a^3 d and an arm for its not being real, and then provided ... and 8 a^3 c - 8 i a^3 d in RR: an answer that holds nowhere, since that discriminant is real only at d = 0.

Part of #718.