Unalias source from dest in copytrito

[jishnub]

Currently, copytrito! doesn't check for aliasing, and as a consequence, if the source and destination overlap, the source is overwritten during the copy, and the destination doesn't receive the original values contained in the source:

julia> M = Matrix(reshape(1:3^2, 3, 3))
3×3 Matrix{Int64}:
 1  4  7
 2  5  8
 3  6  9

julia> A = view(M, 1:size(M,1)-1, 1:size(M,2)-1)
2×2 view(::Matrix{Int64}, 1:2, 1:2) with eltype Int64:
 1  4
 2  5

julia> B = view(M, 2:size(M,1), 2:size(M,2))
2×2 view(::Matrix{Int64}, 2:3, 2:3) with eltype Int64:
 5  8
 6  9

julia> A2 = copy(A);

julia> copytrito!(B, A, 'U')
2×2 view(::Matrix{Int64}, 2:3, 2:3) with eltype Int64:
 1  4
 6  1

julia> UpperTriangular(B) == UpperTriangular(A2)
false

julia> UpperTriangular(B) == UpperTriangular(A)
true

Ideally, copytrito! should behave like copyto!, and unalias the arguments before the actual copy. After this PR,

julia> copytrito!(B, A, 'U')
2×2 view(::Matrix{Int64}, 2:3, 2:3) with eltype Int64:
 1  4
 6  5

julia> UpperTriangular(B) == UpperTriangular(A2)
true

julia> UpperTriangular(B) == UpperTriangular(A)
false

This way, the destination receives the original values in A, irrespective of whether A is updated during the copy.

[dkarrasch]

Linalg tests pass.