C++: fix inconsistency with global var constructor

cpp/ql/lib/semmle/code/cpp/exprs/Expr.qll

@@ -49,6 +49,9 @@ class Expr extends StmtParent, @expr {
   /** Gets the enclosing variable of this expression, if any. */
   Variable getEnclosingVariable() { result = exprEnclosingElement(this) }
 
+  /** Gets the enclosing variable or function of this expression. */
+  Declaration getEnclosingDeclaration() { result = exprEnclosingElement(this) }
+
   /** Gets a child of this expression. */
   Expr getAChild() { exists(int n | result = this.getChild(n)) }