[clang] [clang-tools-extra] [Clang][AST][NFC] Introduce `NamespaceBaseDecl` (PR #149123)

[Christian Kandeler via cfe-commits]

ckandeler wrote:

Not sure how important that is, but I notice that in the AST sent by clangd, namespace aliases now have node kind "Namespace" instead of "NamespaceAlias".

https://github.com/llvm/llvm-project/pull/149123