Add a fully typed Diagonal constructor from AbstractMatrixes (#52487)

The following works after this PR:
```julia
julia> oftype(Diagonal(Float32[1,2]), [1 0; 0 2])
2×2 Diagonal{Float32, Vector{Float32}}:
 1.0   ⋅ 
  ⋅   2.0
```
This changes the behavior of the constructor to copy the diagonal, so
now
```julia
julia> D = Diagonal([1,2]);

julia> typeof(D)(D).diag === D.diag
false
```
whereas this used to be `true` previously. This probably doesn't matter
much, as in most non-trivial cases it'd be copied anyway, and this
conversion is unusual in the trivial case.

---------

Co-authored-by: Daniel Karrasch <daniel.karrasch@posteo.de>

stdlib/LinearAlgebra/src/diagonal.jl

@@ -10,7 +10,6 @@ struct Diagonal{T,V<:AbstractVector{T}} <: AbstractMatrix{T}
         new{T,V}(diag)
     end
 end
-Diagonal{T,V}(d::Diagonal) where {T,V<:AbstractVector{T}} = Diagonal{T,V}(d.diag)
 Diagonal(v::AbstractVector{T}) where {T} = Diagonal{T,typeof(v)}(v)
 Diagonal{T}(v::AbstractVector) where {T} = Diagonal(convert(AbstractVector{T}, v)::AbstractVector{T})
 
@@ -102,6 +101,7 @@ julia> Diagonal(A)
 """
 Diagonal(A::AbstractMatrix) = Diagonal(diag(A))
 Diagonal{T}(A::AbstractMatrix) where T = Diagonal{T}(diag(A))
+Diagonal{T,V}(A::AbstractMatrix) where {T,V<:AbstractVector{T}} = Diagonal{T,V}(diag(A))
 function convert(::Type{T}, A::AbstractMatrix) where T<:Diagonal
     checksquare(A)
     isdiag(A) ? T(A) : throw(InexactError(:convert, T, A))

stdlib/LinearAlgebra/test/diagonal.jl

@@ -50,6 +50,13 @@ Random.seed!(1)
         DI = Diagonal([1,2,3,4])
         @test Diagonal(DI) === DI
         @test isa(Diagonal{elty}(DI), Diagonal{elty})
+
+        # diagonal matrices may be converted to Diagonal
+        local A = [1 0; 0 2]
+        local DA = convert(Diagonal{Float32,Vector{Float32}}, A)
+        @test DA isa Diagonal{Float32,Vector{Float32}}
+        @test DA == A
+
         # issue #26178
         @test_throws MethodError convert(Diagonal, [1,2,3,4])
         @test_throws DimensionMismatch convert(Diagonal, [1 2 3 4])