Blank space.
Hide InterpreterExternalSemaSource class as an implementation detail.
Remove private ctor.
Hide the callback adaptor classes in the implementation.
Subscribe to the interesting events.
For now we will only work/check on declarations that are not deserialized.

We don't want our null deref checker to deserialize the whole PCH/PCM. We should
come up with an idea to work around it.
Fix typo.
Move the PPCallbacks in InterpreterCallbacks too.