[clang] [clang][driver][clang-cl] Fix unused argument warning for `/std:c++20` for precompiled module inputs to `clang-cl` (PR #99300)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

Maybe something like this:

```
// RUN: rm -rf %t
// RUN: split-file %s %t
// RUN: %clang %t/fake.pcm -std=c++20 -### 2>&1 | FileCheck %t/fake.pcm

//--- fake.pcm
// CHECK-NOT: warning
```

https://github.com/llvm/llvm-project/pull/99300