swift234

IRGen: Follow SIL's logic for deciding when overrides reuse vtable slots.

IRGen miscalculated that the allocating constructor for a required init that overrides a non-required init introduces a new vtable entry; the non-required super allocator is statically dispatched. Fix this and head off further divergences by reusing the logic in the new SILDeclRef::getOverriddenVTableEntry method. Fixes rdar://problem/19514920.

Swift SVN r25068