Loading of external comments is expensive for large AST files and not necessary for all use-cases.
The way I understand the intention in ActOnDocumentableDecls is that we are just checking if some of existing comments should be attached to newly created Decls. I assume we don't need to load external comments since those Decls are "local".
// There is at least one comment that not attached to a decl. // Maybe it should be attached to one of these decls?