fix part of #24166: bug in diagonal typevars on the right

[JeffBezanson]

The first problem is a case like this:

(Tuple{T, T, Ref{T}} where T)  <:  (Tuple{S, S, Ref{Q} where Q} where S)

where S is diagonal but T is not. S is more constrained so this should be false. This is fixed by adding a check that a var (T) appearing as the lower bound of a diagonal var (S) must itself be diagonal.

The second problem is this:

(Tuple{T, T} where T>:Int)  <:  (Tuple{S, S} where S>:Int)

Though this seems absurd, the issue is that S's lower bound comes out to Union{T,Int} which we then think is not concrete. Determining whether a lower bound is concrete enough is still an open problem, but I put a band-aid on this by simplifying Union{T,S} given T>:S to T.