[clang] [Clang] Dump minimization hints for namespaces (PR #151534)

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

@JustinStitt thanks for bringing this up. Export declarations were actually handled well before either, I now have another PR that fixes it #151666. It also sets the ground for handling `LinkageSpecDecl`, which we overlooked until now, but that would be yet another follow-up.

re the suggeted patch: we should not call `decls()` as that would potentially cause extra deserializations and we do not want that given that we are running in a low-level deserialization callback and extra deserialization there may cause ripple effects across Clang (performance-wise and surfacing sleeper bugs affecting correctness too) 

https://github.com/llvm/llvm-project/pull/151534