Cache the computation of core toString predicates.

cpp/ql/src/semmle/code/cpp/Element.qll

@@ -47,6 +47,7 @@ pragma[inline]
  */
 class ElementBase extends @element {
   /** Gets a textual representation of this element. */
+  cached
   string toString() { none() }
 
   /**

csharp/ql/src/semmle/code/dotnet/Element.qll

@@ -10,6 +10,7 @@ import semmle.code.csharp.Location
  */
 class Element extends @dotnet_element {
   /** Gets a textual representation of this element. */
+  cached
   string toString() { none() }
 
   /** Gets the location of this element. */

java/ql/src/semmle/code/Location.qll

@@ -84,6 +84,7 @@ class Top extends @top {
   int getNumberOfCommentLines() { numlines(this, _, _, result) }
 
   /** Gets a textual representation of this element. */
+  cached
   string toString() { hasName(this, result) }
 }