Fix stack-protector.rs on LLVM 17

Prevent fill from being (correctly) optimized away by passing the
address of the alloca to black_box.

Author: nikic

tests/ui/abi/stack-protector.rs

@@ -40,6 +40,8 @@ fn vulnerable_function() {
     // Overwrite the on-stack return address with the address of `malicious_code()`,
     // thereby jumping to that function when returning from `vulnerable_function()`.
     unsafe { fill(stackaddr, bad_code_ptr, 20); }
+    // Capture the address, so the write is not optimized away.
+    std::hint::black_box(stackaddr);
 }
 
 // Use an uninlined function with its own stack frame to make sure that we don't