[clang] [llvm] [Clang] Add `__builtin_stack_address` (PR #148281)

[Matt Arsenault via llvm-commits]

================
@@ -2720,6 +2721,28 @@ static SDValue LowerVAARG(SDValue Op, SelectionDAG &DAG) {
       Align(std::min(PtrVT.getFixedSizeInBits(), VT.getFixedSizeInBits()) / 8));
 }
 
+static SDValue LowerSTACKADDRESS(SDValue Op, SelectionDAG &DAG,
+                                 const SparcSubtarget *Subtarget) {
+  SDValue Chain = Op.getOperand(0);
+  EVT VT = Op->getValueType(0);
+  SDLoc DL(Op);
+
+  unsigned SPReg = SP::O6;
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arsenm wrote:

```suggestion
  MCRegister SPReg = SP::O6;
```

https://github.com/llvm/llvm-project/pull/148281