Specialize `fill!` for `Lower`/`UpperTriangular` (#52570)
Only looping over the triangular part provides a performance boost:
```julia
julia> U = UpperTriangular(rand(3000,3000));
julia> @btime zero($U);
23.575 ms (3 allocations: 68.66 MiB) # master
15.739 ms (3 allocations: 68.66 MiB) # PR
```
This is only really applicable when filling a triangular matrix with
zeros, but this has several applications (e.g. imaginary part of a real
matrix).
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Co-authored-by: Daniel Karrasch <daniel.karrasch@posteo.de>
