[clang] [clang][ssaf] Add accessor for `LUNamespace` (PR #195756)

[Balázs Benics via cfe-commits]

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

I feel like this is a bit of an overkill, but you probably have your reasons.
LGTM with nits.

https://github.com/llvm/llvm-project/pull/195756