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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

> @Endilll Should I go ahead with this change anyways?

I would prefer if you can reduce the scope of the PR by avoiding the introduction of `NamespaceDeclBits` (it looks like you do more than just that). But if you feel strongly about it, I don't want to hold you off because of a PR that haven't yet materialized.

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