[clang] [CIR][NFC] Rename AtomicFence to AtomicFenceOp (PR #171248)

[Sirui Mu via cfe-commits]

================
@@ -60,9 +60,9 @@ static RValue emitBuiltinBitOp(CIRGenFunction &cgf, const CallExpr *e,
   return RValue::get(result);
 }
 
-static mlir::Value makeAtomicFenceValue(CIRGenFunction &cgf,
-                                        const CallExpr *expr,
-                                        cir::SyncScopeKind syncScope) {
+static mlir::Value makeAtomicFenceOpValue(CIRGenFunction &cgf,
----------------
Lancern wrote:

Can we make the return type of `makeAtomicFenceOp` to be `void`? And the call sites would become

```cpp
case Builtin::BI__atomic_thread_fence:
  emitAtomicFenceOp(*this, e, cir::SyncScopeKind::System);
  return RValue::get(nullptr);
```

https://github.com/llvm/llvm-project/pull/171248