Cholesky accessors of adjoints should return views

Currently accessing the lower triangular portion of a `Cholesky` object which has been stored using its upper triangle (or vice-versa) will return a copy
https://github.com/JuliaLang/julia/blob/c887c7461b379be7e0bd918f9ffa88a92cb72a97/stdlib/LinearAlgebra/src/cholesky.jl#L342-L350
To avoid unnecessary allocations, you end up with code that looks like this:
https://github.com/JuliaStats/PDMats.jl/blob/eeec1c1bfdddb8f522952f81b3f6a3d5ec8aa7bd/src/pdmat.jl#L57-L59

The logical solution is that it should return an `Adjoint` object. However this would no longer be type-stable, since the flag is stored as a value part of the `Cholesky` struct.

Possible solutions:
a) include the flag as part of the type parameter, either directly as a symbol, or store the `factor` field as a `LowerTriangular`/`UpperTriangular`.
b) get rid of the option and only ever store it as an `UpperTriangular` matrix.

Start at the Cholesky accessor code in stdlib/LinearAlgebra/src/cholesky.jl around lines 342-350, then compare the allocation-avoiding pattern in PDMats.jl src/pdmat.jl around lines 57-59. Evaluate the proposed representation options, with type stability and lower/upper access behavior as acceptance criteria; done means the relevant accessors return views without unnecessary copies.

Written by the indexing model from the issue text.