Reland "Reroute (Upper/Lower)Triangular * Diagonal through __muldiag #55984"

[jishnub]

This relands #55984 which was reverted in #56267. Previously, in #55984, the destination in multiplying triangular matrices with diagonals was also assumed to be triangular, which is not necessarily the case in mul!. Tests for this case, however, were being run non-deterministically, so this wasn't caught by the CI runs.

This improves performance:

julia> U = UpperTriangular(rand(100,100)); D = Diagonal(rand(size(U,2))); C = similar(U);

julia> @btime mul!($C, $D, $U);
  1.517 μs (0 allocations: 0 bytes) # nightly
  1.116 μs (0 allocations: 0 bytes) # This PR