Error if an extern C declaration matches a previous hidden extern C declaration.

Without this patch we produce an error for

extern "C" {
  void f() {
    extern int b;
  }
}
extern "C" {
  extern float b;
}

but not for

extern "C" {
  void f() {
    extern int b;
  }
}
extern "C" {
  float b;
}