Fix issue in JsExportedSource

Model the source as an access to the tainted field, instead of the field itself (which didn't work)

Author: atorralba

swift/ql/lib/codeql/swift/frameworks/StandardLibrary/WebView.qll

@@ -125,7 +125,7 @@ private class JsExportedSource extends RemoteFlowSource {
       base.getEnclosingDecl() instanceof JsExportedProto and
       adopter.getEnclosingDecl() instanceof JsExportedType
     |
-      this.asDefinition().getSourceVariable() = adopter and adopter.getName() = base.getName()
+      this.asExpr().(MemberRefExpr).getMember() = adopter and adopter.getName() = base.getName()
     )
   }
 

swift/ql/test/library-tests/dataflow/flowsources/FlowSources.expected

@@ -11,6 +11,7 @@
 | webview.swift:20:82:20:102 | message | external |
 | webview.swift:25:5:25:13 | .globalObject | external |
 | webview.swift:26:5:26:39 | call to objectForKeyedSubscript(_:) | external |
-| webview.swift:38:10:38:10 | self | Member of a type exposed through JSExport |
-| webview.swift:38:18:38:24 | arg1 | Member of a type exposed through JSExport |
-| webview.swift:38:29:38:35 | arg2 | Member of a type exposed through JSExport |
+| webview.swift:39:9:39:9 | .tainted | Member of a type exposed through JSExport |
+| webview.swift:43:10:43:10 | self | Member of a type exposed through JSExport |
+| webview.swift:43:18:43:24 | arg1 | Member of a type exposed through JSExport |
+| webview.swift:43:29:43:35 | arg2 | Member of a type exposed through JSExport |

swift/ql/test/library-tests/dataflow/flowsources/webview.swift

@@ -31,10 +31,15 @@ protocol Exported : JSExport {
     func tainted(arg1: Any, arg2: Any)
 }
 class ExportedImpl : Exported {
-    var tainted: Any { get { return "" } } // SOURCE
+    var tainted: Any { get { return "" } }
 
     var notTainted: Any { get { return ""} }
 
+    func readFields() {
+        tainted // SOURCE
+        notTainted
+    }
+
     func tainted(arg1: Any, arg2: Any) { // SOURCES
     }