Matmul: `matprod_dest` for `Diagonal` * `SymTridiagonal` (#55039)
We specialize `matprod_dest` for the combination of a `Diagonal` and a
`SymTridiaognal`, in which case the destination is a `Tridiagonal`. With
this, the specialized methods `*(::Diagonal, ::SymTridiagonal)` and
`*(::SymTridiagonal, ::Diagonal)` don't need to be defined anymore,
which reduces potential method ambiguities.
