JS: Add discard predicates for locations

Author: asgerf

javascript/ql/lib/semmle/javascript/internal/Overlay.qll

@@ -28,3 +28,23 @@ overlay[discard_entity]
 private predicate discardEntity(@locatable node) {
   exists(string file | discardableEntity(file, node) and discardFile(file))
 }
+
+overlay[local]
+private string getFileFromLocation(@location loc) {
+  exists(@file file |
+    locations_default(loc, file, _, _, _, _) and
+    files(file, result)
+  )
+}
+
+/** Holds if `loc` is in the `file` and is part of the overlay base database. */
+overlay[local]
+private predicate discardableLocation(string file, @location node) {
+  not isOverlay() and file = getFileFromLocation(node)
+}
+
+/** Holds if `loc` should be discarded, because it is part of the overlay base and is in a file that was also extracted as part of the overlay database. */
+overlay[discard_entity]
+private predicate discardLocation(@location loc) {
+  exists(string file | discardableLocation(file, loc) and discardFile(file))
+}