ldiv! for Diagonal and a sparse vector

[jishnub]

This improves performance:

julia> using LinearAlgebra, SparseArrays

julia> D = Diagonal(rand(3000));

julia> S = sprand(size(D,1), 0.01);

julia> @btime ldiv!($D, $S);
  30.053 μs (0 allocations: 0 bytes) # master
  1.585 μs (0 allocations: 0 bytes) # PR

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.11%. Comparing base (9548149) to head (476c33f).
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #613   +/-   ##
=======================================
  Coverage   84.10%   84.11%           
=======================================
  Files          12       12           
  Lines        9192     9189    -3     
=======================================
- Hits         7731     7729    -2     
+ Misses       1461     1460    -1     

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