Avoid an unnecessary intermediate &mut reference

The `NonNull::as_mut` method returns a mut *reference*, rather than the mut
*pointer* that is intended here.

compiler/rustc_codegen_llvm/src/back/owned_target_machine.rs

@@ -99,7 +99,7 @@ impl Drop for OwnedTargetMachine {
         // llvm::LLVMRustCreateTargetMachine OwnedTargetMachine is not copyable so there is no
         // double free or use after free.
         unsafe {
-            llvm::LLVMRustDisposeTargetMachine(self.tm_unique.as_mut());
+            llvm::LLVMRustDisposeTargetMachine(self.tm_unique.as_ptr());
         }
     }
 }