Improve enumerateDirectSupertypes so that for T? it will return U? if
U is a supertype of T. This is another form of direct supertype.
This is making up for a deficiency in the completeness of our Type
join implementation, which should be able to directly compute a join
of disparate types that share some common supertype, but sometimes
fails to in cases involving protocol compositions.
Fixes rdar://problem/45490737
