[clang] [clang-tools-extra] [Clang][AST] Move NamespaceDecl bits to DeclContext (PR #98567)

[Matheus Izvekov via cfe-commits]

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

LGTM, as a clean up, and I see no notable performance implications.

Is there any follow up work for this?

https://github.com/llvm/llvm-project/pull/98567