not supporting `Diagonal` with lazy `diag`

[putianyi889]

JuliaLang/LinearAlgebra.jl#1158

julia> A=Diagonal(1:5)
5×5 Diagonal{Int64, UnitRange{Int64}}:
 1  ⋅  ⋅  ⋅  ⋅
 ⋅  2  ⋅  ⋅  ⋅
 ⋅  ⋅  3  ⋅  ⋅
 ⋅  ⋅  ⋅  4  ⋅
 ⋅  ⋅  ⋅  ⋅  5

julia> convert_eltype(Float64,A)
ERROR: MethodError: Cannot `convert` an object of type Vector{Int64} to an object of type StepRangeLen{Float64, Base.TwicePrecision{Float64}, Base.TwicePrecision{Float64}, Int64}
The function `convert` exists, but no method is defined for this combination of argument types.