Add default constructor for Reverse{Fwd}()

[oxinabox]

Follow up to #33736
if you have a RevereOrdering type and it has its type parameter.
Then most often it is the standard typeof(Base.Reverse) which is ReverseOrdering{ForwardOrdering}
which can be constructed as ReverseOrdering(ForwardOrdering()) == Reverse

[LilithHafner]

I'm having a hard time picturing a use case for this.

[oxinabox]

It's been two years and I struggle to even remember the PR.
But it was going to let us delete this line: https://github.com/JuliaCollections/DataStructures.jl/pull/687/files#diff-e8c1224688809598c61cad8d4acf12db5a97050a610c0d7cefda0615339a2ccfR55-R57

because just like for the Base.ForwardOrdering type, one would be able to construct the instance for Base.ReverseOrdering{Forward} by just calling () on it
I think

[LilithHafner]

Seems sensible; I wonder if we should support this for all singleton types

This PR is ever-so-slightly stronger than supporting T() for all singleton types T because it also supports ReverseOrdering{T} for all types T with T()defined, so it would be technically breaking to revert this should #47853 be accepted.