Ensure that buffers containing Clang swift_attr attributes are parsed as attributes

Previously, they were being parsed as top-level code, which would cause
errors because there are no definitions. Introduce a new
GeneratedSourceInfo kind to mark the purpose of these buffers so the
parser can handle them appropriately.

tools/SourceKit/lib/SwiftLang/SwiftEditor.cpp

@@ -972,6 +972,7 @@ public:
       case GeneratedSourceInfo::DefaultArgument:
       case GeneratedSourceInfo::ReplacedFunctionBody:
       case GeneratedSourceInfo::PrettyPrinted:
+      case GeneratedSourceInfo::Attribute:
         break;
       }
     }