[APINotes] Initial support for C++ namespaces

This upstreams a part of the C++ namespaces support in Clang API Notes.

The complete patch was recently merged downstream in the Apple fork: https://github.com/apple/llvm-project/pull/7230.

This patch only adds the parts of the namespace support that can be cleanly applied on top of the API Notes infrastructure that was upstreamed previously.

Differential Revision: https://reviews.llvm.org/D159092