I'd suggest to provide default implementations for ExternalASTSource's
pure virtual functions. Derived classes should actually not be required
to implement them. See e.g. r126387 (offering an orthogonal lookup
source through ExternalASTSource::LookupUnqualified()), and lldb.
Attached patch does that (+ cleanup of forward declarations).
I think it's a semi-minor patch, so if there are no comments I'll commit
it in 24 hours
ExternalASTSource_default_impl.diff (3.73 KB)