[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

include/llvm/CodeGen/MachineFrameInfo.h

@@ -516,10 +516,6 @@ public:
   /// on the stack.  Returns an index with a negative value.
   int CreateFixedSpillStackObject(uint64_t Size, int64_t SPOffset);
 
-  /// Allocates memory at a fixed, target-specific offset from the frame
-  /// pointer. Marks the function as having its frame address taken.
-  int CreateFrameAllocation(uint64_t Size);
-
   /// isFixedObjectIndex - Returns true if the specified index corresponds to a
   /// fixed stack object.
   bool isFixedObjectIndex(int ObjectIdx) const {