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

[Nico Weber via cfe-commits]

https://github.com/nico approved this pull request.

Hm, maybe it's less misleading of we print a "don't know this flag" diag rather than giving the impression that your code builds fine with the edg frontend 😅 But code looks fine, feel free to land if you think accepting the flag is better.

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