C++: Add a few helper predicates.

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

@@ -192,6 +192,11 @@ class Element extends ElementBase {
    */
   predicate isAffectedByMacro() { affectedByMacro(this) }
 
+  /** Holds if this element is affected by the expansion of `mi`. */
+  predicate isAffectedByMacro(MacroInvocation mi) {
+    affectedbymacroexpansion(underlyingElement(this), unresolveElement(mi))
+  }
+
   private Element getEnclosingElementPref() {
     enclosingfunction(underlyingElement(this), unresolveElement(result)) or
     result.(Function) = stmtEnclosingElement(this) or

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

@@ -23,7 +23,7 @@ class Expr extends StmtParent, @expr {
   predicate hasChild(Expr e, int n) { e = this.getChild(n) }
 
   /** Gets the enclosing function of this expression, if any. */
-  Function getEnclosingFunction() { result = exprEnclosingElement(this) }
+  override Function getEnclosingFunction() { result = exprEnclosingElement(this) }
 
   /** Gets the nearest enclosing set of curly braces around this expression in the source, if any. */
   BlockStmt getEnclosingBlock() { result = this.getEnclosingStmt().getEnclosingBlock() }

cpp/ql/lib/semmle/code/cpp/stmts/Stmt.qll

@@ -20,7 +20,7 @@ class Stmt extends StmtParent, @stmt {
   predicate hasChild(Element e, int n) { this.getChild(n) = e }
 
   /** Gets the enclosing function of this statement, if any. */
-  Function getEnclosingFunction() { result = stmtEnclosingElement(this) }
+  override Function getEnclosingFunction() { result = stmtEnclosingElement(this) }
 
   /**
    * Gets the nearest enclosing block of this statement in the source, if any.
@@ -159,7 +159,10 @@ private class TStmtParent = @stmt or @expr;
  *
  * This is normally a statement, but may be a `StmtExpr`.
  */
-class StmtParent extends ControlFlowNode, TStmtParent { }
+class StmtParent extends ControlFlowNode, TStmtParent {
+  /** Gets the enclosing function of this element, if any. */
+  Function getEnclosingFunction() { none() }
+}
 
 /**
  * A C/C++ 'expression' statement.