[clang] [PAC] Implement authentication for C++ member function pointers (PR #99576)

[Daniil Kovalev via cfe-commits]

================
@@ -853,6 +877,25 @@ llvm::Value *ItaniumCXXABI::EmitMemberDataPointerAddress(
                                    "memptr.offset");
 }
 
+// See if it's possible to return a constant signed pointer.
+static llvm::Constant *pointerAuthResignConstant(
+    llvm::Value *Ptr, const CGPointerAuthInfo &CurAuthInfo,
+    const CGPointerAuthInfo &NewAuthInfo, CodeGenModule &CGM) {
+  auto *CPA = dyn_cast<llvm::ConstantPtrAuth>(Ptr);
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kovdan01 wrote:

Nit

```suggestion
  const auto *CPA = dyn_cast<llvm::ConstantPtrAuth>(Ptr);
```

https://github.com/llvm/llvm-project/pull/99576