`sol.prob` on an `OptimizationSolution` gives `OptimizationCache` instead of `OptimizationProblem`

[jClugstor]

Describe the bug 🐞

I'm not sure if this is intended, but the prob field in OptimizationProblems hold the OptimizationCache, if the algorithm is one that supports caches.

Expected behavior

I would expect that calling sol.prob I would get the original OptimizationProblem back.

As it is, it's almost impossible to get the original problem from the solution. OptimizationCache stores versions of the constraint functions with p fixed. Concretely, this makes OptimizationAdjoint have to look inside of the closure made when p is fixed in the OptimizationCache, which is very brittle.