When computing an overlap between a property-like rule (T.[p] => T for some
property symbol [p]) and another rule, try harder to ensure that the new
rule is a property-like rule.
In a conformance-valid rewrite system, all rules that are not LHS- or RHS-
simplified will eventually either be property-like or same-type rules, but
we need to maintain this invariant for that rules that become simplified
as well, to ensure that rewrite loops have a certain structure that is
important for the minimal conformances algorithm.
I don't quite understand why to be honest, but I'm close to figuring it
out.
Fixes rdar://problem/91232987.
