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

[Krystian Stasiowski via cfe-commits]

sdkrystian wrote:

> Is there any follow up work for this?

@mizvekov Yes, but it's not directly related. I'm going to submit a patch in the future which stores the `SourceLocation` of the `inline` keyword for inline namespace, and perhaps another patch that extends `NestedNameSpecifier` such that it can represent a _nested-namespace-definition_.

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