concatenations involving special matrices yield dense matrices

[Sacha0]

Ref. #17660. Concatenations involving special matrices yield dense matrices, e.g.

julia> hcat(Diagonal(1:4), Diagonal(1:4))
4×8 Array{Int64,2}:
 1  0  0  0  1  0  0  0
 0  2  0  0  0  2  0  0
 0  0  3  0  0  0  3  0
 0  0  0  4  0  0  0  4

julia> hcat(sprand(4, 4, 0.5), Diagonal(1:4))
4×8 Array{Float64,2}:
 0.638976  0.0  0.0       0.46798   1.0  0.0  0.0  0.0
 0.0       0.0  0.0       0.0       0.0  2.0  0.0  0.0
 0.0       0.0  0.906125  0.172851  0.0  0.0  3.0  0.0
 0.0       0.0  0.508749  0.484722  0.0  0.0  0.0  4.0

whereas they should yield sparse matrices. Best!