Fix negation for an immutable unit triangular (#52510)
Fixes
```julia
julia> using FillArrays, LinearAlgebra
julia> U = UnitUpperTriangular(Fill(2,4,4))
4×4 UnitUpperTriangular{Int64, Fill{Int64, 2, Tuple{Base.OneTo{Int64}, Base.OneTo{Int64}}}}:
1 2 2 2
⋅ 1 2 2
⋅ ⋅ 1 2
⋅ ⋅ ⋅ 1
julia> -U
ERROR: ArgumentError: Cannot setindex! to -1 for an AbstractFill with value -2.
Stacktrace:
[1] setindex!
@ ~/.julia/packages/FillArrays/oXkMk/src/FillArrays.jl:52 [inlined]
[2] -(A::UnitUpperTriangular{Int64, Fill{Int64, 2, Tuple{Base.OneTo{Int64}, Base.OneTo{Int64}}}})
@ LinearAlgebra ~/packages/julias/julia-latest/share/julia/stdlib/v1.11/LinearAlgebra/src/triangular.jl:442
[3] top-level scope
@ REPL[33]:1
```
After this,
```julia
julia> -U
4×4 UpperTriangular{Int64, Matrix{Int64}}:
-1 -2 -2 -2
⋅ -1 -2 -2
⋅ ⋅ -1 -2
⋅ ⋅ ⋅ -1
```
