[SourceKit] Ignore the '__raw_doc_comment' attribute, which is more of an implementation detail.

tools/SourceKit/lib/SwiftLang/SwiftLangSupport.cpp

@@ -656,6 +656,7 @@ std::vector<UIdent> SwiftLangSupport::UIDsFromDeclAttributes(const DeclAttribute
     case DAK_SetterAccessibility:
     // Ignore these.
     case DAK_ShowInInterface:
+    case DAK_RawDocComment:
       continue;
     default:
       break;