C++: Compute locations only for elements in result

This gains some performance by not computing locations for all
expressions since we are only interested in calls and variable accesses.
The `Top::hasLocationInfo` predicate goes from 2m28s to 1m32s on
Chromium.

cpp/ql/src/definitions.qll

@@ -30,7 +30,7 @@ class Top extends Element {
   predicate hasLocationInfo(string filepath,
                             int startline, int startcolumn,
                             int endline, int endcolumn) {
-    this instanceof Element and
+    interestingElement(this) and
     not this instanceof MacroAccess and
     not this instanceof Include and
     exists(Location l |
@@ -76,6 +76,13 @@ predicate hasLocationInfo_Include(Include i, string path, int sl, int sc, int el
   )
 }
 
+/** Holds if `e` is a source or a target of jump-to-definition. */
+predicate interestingElement(Element e) {
+  exists(definitionOf(e, _))
+  or
+  e = definitionOf(_, _)
+}
+
 /**
  * Holds if `f`, `line`, `column` indicate the start character
  * of `cc`.