do not replace LazyBufferCache when promoting f

[thomvet]

Addresses SciML/OrdinaryDiffEq.jl#2719
Needs: SciML/PreallocationTools.jl#131 and SciML/SciMLBase.jl#1097

[ChrisRackauckas]

Why would an LBF be promoted?

[thomvet]

It shouldn't be, but when I am running the example in SciML/OrdinaryDiffEq.jl#2719, promotion is attempted and this was a way of safe-guarding against this. I can look for a more elegant solution.

[ChrisRackauckas]

It should be using DiffCache now?

[thomvet]

Yes, with the newest version of SciML/SciMLBase.jl#1097 that should be the case.

We just need the no-op (promote_f(f::SplitFunction, ::Val{specialize}, u0, p, t) where {specialize} = f) here to keep the DiffCache in place, otherwise it gets converted into a zero(u0), which just re-creates the original problem of not having the right cache.

[thomvet]

This is still needed to make SciML/SciMLBase.jl#1106 work (see the test failure of splitodeproblem_cache.jl - without the change here the DiffCache just gets removed when promote_f is called).