`copyto!` of triangular of adjoint fails

```julia
julia> using CUDA, LinearAlgebra

julia> CUDA.allowscalar(false)

julia> U = cu(UpperTriangular(
[1.0 0.0;
0.0 1.0]
))
2×2 UpperTriangular{Float32, CuArray{Float32, 2}}:
1.0 0.0
⋅ 1.0

julia> copyto!(Σ, U')
ERROR: scalar getindex is disallowed
Stacktrace:
[1] error(s::String)
@ Base ./error.jl:33
[2] assertscalar(op::String)
@ GPUArrays ~/.julia/packages/GPUArrays/4n0iS/src/host/indexing.jl:62
[3] getindex(::CuArray{Float32, 2}, ::Int64, ::Int64)
@ GPUArrays ~/.julia/packages/GPUArrays/4n0iS/src/host/indexing.jl:104
[4] getindex
@ /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/LinearAlgebra/src/adjtrans.jl:203 [inlined]
[5] getindex
@ /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/LinearAlgebra/src/triangular.jl:222 [inlined]
[6] _getindex
@ ./abstractarray.jl:1214 [inlined]
[7] getindex
@ ./abstractarray.jl:1170 [inlined]
[8] iterate
@ ./abstractarray.jl:1096 [inlined]
[9] iterate
@ ./abstractarray.jl:1094 [inlined]
[10] copyto_unaliased!(deststyle::IndexLinear, dest::CuArray{Float32, 2}, srcstyle::IndexCartesian, src::LowerTriangular{Float32, Adjoint{Float32, CuArray{Float32, 2}}})
@ Base ./abstractarray.jl:975
[11] copyto!(dest::CuArray{Float32, 2}, src::LowerTriangular{Float32, Adjoint{Float32, CuArray{Float32, 2}}})
@ Base ./abstractarray.jl:950
[12] top-level scope
@ REPL[11]:1
[13] top-level scope
@ ~/.julia/packages/CUDA/LTbUr/src/initialization.jl:81

julia> @which copyto!(Σ, U')
copyto!(dest::AbstractArray, src::AbstractArray) in Base at abstractarray.jl:947
```
Type is
```julia
julia> typeof(U')
LowerTriangular{Float32, Adjoint{Float32, CuArray{Float32, 2}}}
```
which means that it won't hit https://github.com/JuliaGPU/GPUArrays.jl/blob/8e1bde0ee96b3da889ba99af4caddbecedcfd0cc/src/host/linalg.jl#L70 because `Adjoint` is not a subtype of `AbstractGPUArray`, nor will it hit https://github.com/JuliaGPU/GPUArrays.jl/blob/8e1bde0ee96b3da889ba99af4caddbecedcfd0cc/src/host/linalg.jl#L70 since this is a `LowerTriangular`.

Similar issue of course also exists for `UpperTriangular`.

Hotfix would be to just add additional impl for `UpperTriangular{T, <:Adjoint{T, <:AbstractGPUArray{T,N}}}`, etc. but unclear to me if this is the best approach, e.g. maybe there's a more "generic" solution to these wrappers of wrappers arrays?

Reproduce the failure with the Julia example and inspect the copyto! dispatches in src/host/linalg.jl, especially the implementation around the referenced line. Compare the LowerTriangular and Adjoint wrapper types involved in the Base copyto! path. Done means copying the adjoint triangular GPU array without scalar getindex errors, with equivalent behavior for the related triangular case.

Written by the indexing model from the issue text.