fix #7933, full(Symmetric(A)) mutates A

This could potentially be improved in terms of performance by not copying the elements of `A.S` that we know are going to be overwritten by `copytri!`. Though currently `full()` on a Triangular matrix doesn't worry about that, see https://github.com/JuliaLang/julia/blob/793af3fc2f58b2f16c68da641fb34702c95af41e/base/linalg/triangular.jl#L102-L112