Honoring constraint on parametric type's parameter in UnionAll type

[wsshin]

Consider the following definition of a parametric type named MyType:

julia> VERSION
v"1.6.1-pre.1"

julia> struct MyType{R<:Real}
           x::R
       end

Now, we get

julia> (MyType{R} where {R<:Real}) <: MyType
true

but

julia> (MyType{R} where {R}) <: MyType
false

Because the parameter R of MyType must be a subtype of Real by definition, it will be nice if the last code returns true.