Revert "Recommit DwarfEHPrepare: insert extra unwind paths for stack protector to instrument"

This broke Objective-C autorelease / retainAutoreleasedReturnValue, see
comments on the code review.

> This is a mitigation patch for
> https://bugs.chromium.org/p/llvm/issues/detail?id=30, where existing stack
> protection is skipped if a function is returned through by an unwinder rather
> than the normal call/return path. The recent patch D139254 added the ability to
> instrument a visible unwind path, at least in the IR case (I'm working on the
> SelectionDAG instrumentation too) but there are still invisible unwinds it
> can't reach.
>
> So this patch adds logic to DwarfEHPrepare that goes through a function,
> converting any call that might throw into an invoke to a simple resume cleanup,
> and adding cleanup clauses to existing landingpads that lack them. Obviously we
> don't really want to do this if it's wasted effort, so I also exposed
> requiresStackProtector from the actual StackProtector code to skip the extra
> paths if they won't be used.
>
> Changes:
>   * Move test to AArch64 directory as it relies on target presence.
>   * Re-add Dominator-tree maintenance. Accidentally cherry-picked wrong patch.
>   * Skip adding paths on Windows EH functions.
>
> https://reviews.llvm.org/D143637

This reverts commit 2d690684.