[llvm] [NFC] Try to unbreak the module builds due to missing `StringRef.h` include in `UniqueBBID.h` (PR #191877)

[Dan Liew via llvm-commits]

delcypher wrote:

Linux failure looks completely unrelated:

```
2026-04-13T19:29:12.5319472Z ********************
2026-04-13T19:29:12.5319663Z Failed Tests (1):
2026-04-13T19:29:12.5319919Z   lldb-api :: python_api/run_locker/TestRunLocker.py
2026-04-13T19:29:12.5320133Z 
```

Going to merge now.

https://github.com/llvm/llvm-project/pull/191877