Parameter.qll: Tweak how effective declaration entries are computed

With the new formulation, we can join on function and index at the
same time, leading to significant performance gains on large code
bases that use templates extensively.

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

@@ -68,10 +68,9 @@ class Parameter extends LocalScopeVariable, @parameter {
    */
   private VariableDeclarationEntry getAnEffectiveDeclarationEntry() {
     if getFunction().isConstructedFrom(_)
-      then exists (Parameter prototype
-           | prototype = result.getVariable() and
-             prototype.getIndex() = getIndex() and
-             getFunction().isConstructedFrom(prototype.getFunction()))
+      then exists (Function prototypeInstantiation
+           | prototypeInstantiation.getParameter(getIndex()) = result.getVariable() and
+             getFunction().isConstructedFrom(prototypeInstantiation))
       else result = getADeclarationEntry()
   }