Avoid inconsistent implicit toString on potential string subtypes

Fix cases where the compiler cannot prove types don't extend string:

1. FileSystem.qll: Add toString() to ContainerBase signature. The
   instantiation sites already provide toString(), so no changes needed
   there. Container.toString() now delegates to super.toString().
   No behaviour change.

2. DataFlowImplCommon.qll: Content.toString() now delegates to
   super.toString() instead of returning the literal "Content".
   This is a behaviour change: Content now displays its actual
   description (e.g. field names) rather than the generic "Content".

shared/dataflow/codeql/dataflow/internal/DataFlowImplCommon.qll

@@ -112,7 +112,7 @@ module MakeImplCommon<LocationSig Location, InputSig<Location> Lang> {
     final private class LangContentSet = Lang::ContentSet;
 
     class Content extends LangContentSet {
-      string toString() { result = "Content" }
+      string toString() { result = super.toString() }
     }
 
     class ContentFilter extends Unit {

shared/util/codeql/util/FileSystem.qll

@@ -23,6 +23,13 @@ signature module InputSig {
      * Typically `containerparent(result, this)`.
      */
     ContainerBase getParentContainer();
+
+    /**
+     * Gets a textual representation of this container.
+     *
+     * Typically `result = this.getAbsolutePath()`.
+     */
+    string toString();
   }
 
   /**
@@ -206,7 +213,7 @@ module Make<InputSig Input> {
      *
      * This is the absolute path of the container.
      */
-    string toString() { result = this.getAbsolutePath() }
+    string toString() { result = super.toString() }
   }
 
   /** A folder. */