[clang] [Clang] Assert non-null enum definition in CGDebugInfo (PR #105556)

[via cfe-commits]

smanna12 wrote:

> Per discussion in #97105, this looks good to me.
> 
> However, please do update the PR title and adjust the commit message accordingly when merging the change as requested by @pogo59.

Thanks @tahonermann and @pogo59 for reviews.

I have updated title and commit messages.

https://github.com/llvm/llvm-project/pull/105556