[clang] [CIR][NFC] Fix warning about unhandled Decl (PR #140159)

[Erich Keane via cfe-commits]

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

Is this fixing this warning? 

```
/local/home/ekeane/llvm-project/clang/lib/CIR/CodeGen/CIRGenDecl.cpp:270:11: warning: enumeration value 'HLSLRootSignature' not handled in switch [-Wswitch]
```

https://github.com/llvm/llvm-project/pull/140159