[llvm] [Xtensa] Implement vararg support. (PR #117126)

[Matt Arsenault via llvm-commits]

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@@ -25,13 +25,14 @@ class XtensaMachineFunctionInfo : public MachineFunctionInfo {
   /// FrameIndex of the spill slot for the scratch register in BranchRelaxation.
   int BranchRelaxationScratchFrameIndex = -1;
   unsigned VarArgsFirstGPR;
-  int VarArgsStackOffset;
-  unsigned VarArgsFrameIndex;
+  unsigned VarArgsOnStackFrameIndex;
+  unsigned VarArgsInRegsFrameIndex;
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arsenm wrote:

```suggestion
  int VarArgsInRegsFrameIndex;
```

Frame indexes are signed

https://github.com/llvm/llvm-project/pull/117126