[llvm] [BOLT][binary-analysis] Add initial pac-ret gadget scanner (PR #122304)

[Anatoly Trosinenko via llvm-commits]

================
@@ -550,6 +550,22 @@ class MCPlusBuilder {
     return Analysis->isReturn(Inst);
   }
 
+  virtual MCPhysReg getAuthenticatedReg(const MCInst &Inst) const {
+    llvm_unreachable("not implemented");
+    return false;
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atrosinenko wrote:

```suggestion
    return getNoRegister();
```

https://github.com/llvm/llvm-project/pull/122304