[clang] [clang-cl] Parse (but don't support) /BE (PR #228381)

[Hans Wennborg via cfe-commits]

zmodem wrote:

To clarify, this makes clang-cl parse it, not claim to support it. Before this it would print:

```
error: no such file or directory: '/BE'
```

now it will print:

```
warning: argument unused during compilation: '/BE' [-Wunused-command-line-argument]
```

https://github.com/llvm/llvm-project/pull/228381