[WinEH] Sink UnwindHelp completely out of IR

[WinEH] Sink UnwindHelp completely out of IR

We don't need to represent UnwindHelp in IR.  Instead, we can use the
knowledge that we are emitting the parent function to decide if we
should create the UnwindHelp stack object.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@234061 91177308-0d34-0410-b5e6-96231b3b80d8