[Asan] Fix -Wunused-private-field in non-assertion builds (NFC)

llvm-project/llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp:650:13:
error: private field 'OwnerFn' is not used [-Werror,-Wunused-private-field]
  Function *OwnerFn = nullptr;
            ^
1 error generated.

Author: DamonFool

llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp

@@ -656,6 +656,7 @@ class RuntimeCallInserter {
                               ArrayRef<Value *> Args = {},
                               const Twine &Name = "") {
     assert(IRB.GetInsertBlock()->getParent() == OwnerFn);
+    (void)OwnerFn;
     return IRB.CreateCall(Callee, Args, Name, nullptr);
   }
 };