[Fiber] Push class context providers even if they crash (#8627)

* Push class context providers early Previously we used to push them only after the instance was available. This caused issues in cases an error is thrown during componentWillMount(). In that case we never got to pushing the provider in the begin phase, but in complete phase the provider check returned true since the instance existed by that point. As a result we got mismatching context pops. We solve the issue by making the context check independent of whether the instance actually exists. Instead we're checking the type itself. This lets us push class context early. However there's another problem: we might not know the context value. If the instance is not yet created, we can't call getChildContext on it. To fix this, we are introducing a way to replace current value on the stack, and a way to read the previous value. This also helps remove some branching and split the memoized from invalidated code paths. * Add a now-passing test from #8604 Also rename another test to have a shorter name. * Move isContextProvider() checks into push() and pop() All uses of push() and pop() are guarded by it anyway. This makes it more similar to how we use host context. There is only one other place where isContextProvider() is used and that's legacy code needed for renderSubtree(). * Clarify why we read the previous context * Use invariant instead of throwing an error * Fix off-by-one in ReactFiberStack * Manually keep track of the last parent context The previous algorithm was flawed and worked by accident, as shown by the failing tests after an off-by-one was fixed. The implementation of getPrevious() was incorrect because the context stack currently has no notion of a previous value per cursor. Instead, we are caching the previous value directly in the ReactFiberContext in a local variable. Additionally, we are using push() and pop() instead of adding a new replace() method.