[clang] [clang][ASTImporter] Fix AST import if anonymous namespaces are merged (PR #128735)

[Balázs Kéri via cfe-commits]

balazske wrote:

I meant that I did not verify where a namespace declaration is allowed, if it is allowed with another declaration as parent (which is not a `LinkageSpecDecl` and not `NamespaceDecl` and not `TranslationUnitDecl`). It looks likely that no other parent is possible.

https://github.com/llvm/llvm-project/pull/128735