C++: Use isConstexpr instead of workaround

The `addressConstantVariable` predicate was the slowest single predicate
when running the full LGTM suite on Chromium. Fortunately it's only
executed once, but it could be easily made faster by using the new
`Variable.isConstexpr` predicate instead of the slow workaround that was
in its place.

cpp/ql/src/semmle/code/cpp/internal/AddressConstantExpression.qll

@@ -13,10 +13,7 @@ predicate addressConstantExpression(Expr e) {
 /** Holds if `v` is a constexpr variable initialized to a constant address. */
 private predicate addressConstantVariable(Variable v) {
   addressConstantExpression(v.getInitializer().getExpr().getFullyConverted()) and
-  // Here we should also require that `v` is constexpr, but we don't have that
-  // information in the db. See CPP-314. Instead, we require that the variable
-  // is never defined except in its initializer.
-  forall(Expr def | definition(v, def) | def = any(Initializer init).getExpr())
+  v.isConstexpr()
 }
 
 /**