How to insert statements at the beginning of a function block using Clang AST?

Question

I am writing a source to source transformer using clang. I want to insert one flag variable for each parameter in a function. So if my original source is like below:

int f(int x, int y)
{
    // do something ...
    return 0;
}

I want to transform it to:

int f(int x, int y)
{
    bool __flag_x = true;
    bool __flag_y = true;
    // do something ...
    return 0
}

Instead I am getting:

int f(int x, int y)
bool __flag_x = true;
bool __flag_y = true;
{
    // do something ...
    return 0
}

The problem is, it is inserting the params before the left curly braces.

How do I insert it just after the brace?

Here is my AST matcher/rewriter:

  bool VisitFunctionDecl(FunctionDecl *func) {
    for (unsigned int i = 0; i < func->getNumParams(); i++) {
      std::string varString = func->parameters()[i]->getQualifiedNameAsString();
      TheRewriter.InsertText(func->getBody()->getBeginLoc(),
                             "bool __flag_" + varString + " = true;\n");
    }
    return true;
  }