Retcode not correctly set

```julia
A = rand(4, 4)
b = rand(4)
prob = LinearProblem(A, b)
sol1 = solve(prob)
sol2 = solve(prob, KrylovJL_CG())
```

Problem: ReturnCode is Default
Expectation: ReturnCode should be Success (for sol1) or Failure/MaxIters (for sol2)

```julia
A = zeros(4, 4)
b = rand(4)
prob = LinearProblem(A, b)
sol = solve(prob)
```

Works as expected.