[AST] Add a const version of CallExpr::children() (NFC)

[vsk]

Summary

This makes it possible to visit the children of a CallExpr from a ConstStmtVisitor.

[vsk]

I no longer need this functionality.

include/clang/AST/Expr.h

            T->getStmtClass() <= lastCallExprConstant;
   }
 
   // Iterators
   child_range children() {
     return child_range(&SubExprs[0],
                        &SubExprs[0]+NumArgs+getNumPreArgs()+PREARGS_START);
   }
+  const_child_range children() const {
+    return const_child_range(const_cast<CallExpr *>(this)->children());
+  }
 };
 
 /// Extra data stored in some MemberExpr objects.
 struct MemberExprNameQualifier {
   /// \brief The nested-name-specifier that qualifies the name, including
   /// source-location information.
   NestedNameSpecifierLoc QualifierLoc;