Fix fetching parent in symmetric algebra

[jishnub]

We only need the parent of the result if it is a triangular matrix. For other structurally triangular matrices, such as Diagonals, we may use these directly in the Hermitian constructor.

The operation proceeds as (assuming H isa Hermitian with H.uplo == 'U'):

function +(H::Hermitian, H::Hermitian)
    U = uppertriangular(parent(H))
    Ures = U + U
    data = Ures isa UpperTriangular ? parent(Ures) : Ures # this PR
    Hermitian(data, :U)
end

This accounts for the fact that Ures may not be an UpperTriangular, as uppertriangular might be specialized by the parent. In such cases we should not extract the parent. Currently, only Diagonal specializes uppertriangular, so this issue only exists for a Hermitian or a Symmetric that wraps a Diagonal.

Fixes #56283