[SSAF] Group virtual method slots into override families
A virtual call site can dispatch to any override, so the parameter and return
slots that occupy the same vtable slot across an override chain are
indistinguishable to a caller. Whole-program consumers therefore have to treat
them as one unit or they will reason about a slot that a call never actually
reaches.
Compute those units up front, keyed per slot, so consumers only need a map
lookup rather than their own traversal of the override relation. Overloads
occupy distinct vtable slots and stay in distinct families.
The family representative is the smallest EntityId in the class, which keeps
the result stable across runs.
ยง2 of rdar://179151603
