Generate native unwind info on Win64

This patch enables LLVM to emit Win64-native unwind info rather than
DWARF CFI.  It handles all corner cases (I hope), including stack
realignment.

Because the unwind info is not flexible enough to describe stack frames
with a gap of unknown size in the middle, such as the one caused by
stack realignment, I modified register spilling code to place all spills
into the fixed frame slots, so that they can be accessed relative to the
frame pointer.

Patch by Vadim Chugunov!

Reviewed By: rnk

Differential Revision: http://reviews.llvm.org/D4081

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211399 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/MachineFrameInfo.h

@@ -484,6 +484,9 @@ public:
   ///
   int CreateFixedObject(uint64_t Size, int64_t SPOffset, bool Immutable);
 
+  /// CreateFixedSpillStackObject - Create a spill slot at a fixed location
+  /// on the stack.  Returns an index with a negative value.
+  int CreateFixedSpillStackObject(uint64_t Size, int64_t SPOffset);
 
   /// isFixedObjectIndex - Returns true if the specified index corresponds to a
   /// fixed stack object.