[clang] [Clang][Attr] Fix possible crash when trying to check for DeviceKernel spelling (PR #143546)

[Mariya Podchishchaeva via cfe-commits]

Fznamznon wrote:

> I didn't add a test because this can't be reproduced yet in this repo, I reproduced this only in intel's fork with more SYCL support.

But that does sound specific to the downstream, right? Do we observe the issue there because intel's fork used to add OpenCLKernelAttr implicitly in SemaSYCL?

https://github.com/llvm/llvm-project/pull/143546