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

[Andy Kaylor via cfe-commits]

andykaylor wrote:

> 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]
> ```

Yes

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