[llvm] [llvm-extract] support unnamed bbs. (PR #135140)

[Allin Lee via llvm-commits]

AllinLeeYL wrote:

> This looks handy 🙂 Is it worth adding a small test in `llvm/test/tools/llvm-extract`?

Thank you for your advice🙏. A test has been added to the latest commit. 

https://github.com/llvm/llvm-project/pull/135140