Avoid accessing unset indices in symmetric copyto!

[jishnub]

Currently, copying Symmetric/Hermitian matrices with mismatched uplos might access unset indices. In this PR, we fix this by ensuring that we always use copytrito!, and only access the triangular half of the parent that uplo corresponds to.
On master,

julia> using LinearAlgebra

julia> M = Matrix{Complex{BigFloat}}(undef, 2, 2)
2×2 Matrix{Complex{BigFloat}}:
 #undef  #undef
 #undef  #undef

julia> M[1,1] = M[2,2] = 3
3

julia> uplo1, uplo2 = :U, :L
(:U, :L)

julia> isupper = uplo1 == :U
true

julia> M[1+!isupper, 1+isupper] = 4+3im
4 + 3im

julia> M
2×2 Matrix{Complex{BigFloat}}:
 3.0+0.0im  4.0+3.0im
  #undef    3.0+0.0im

julia> H1 = Hermitian(M, uplo1)
2×2 Hermitian{Complex{BigFloat}, Matrix{Complex{BigFloat}}}:
 3.0+0.0im  4.0+3.0im
 4.0-3.0im  3.0+0.0im

julia> H2 = Hermitian(M', uplo2)
2×2 Hermitian{Complex{BigFloat}, Adjoint{Complex{BigFloat}, Matrix{Complex{BigFloat}}}}:
 3.0+0.0im  4.0+3.0im
 4.0-3.0im  3.0+0.0im

julia> copyto!(H1, H2)
ERROR: UndefRefError: access to undefined reference
[...]

After this PR,

julia> copyto!(H1, H2)
2×2 Hermitian{Complex{BigFloat}, Matrix{Complex{BigFloat}}}:
 3.0+0.0im  4.0+3.0im
 4.0-3.0im  3.0+0.0im