[clang-tidy] [With reproducible example] Erroneous rule "llvm-header-guard" since v21.0.0

All my header guards raise issues now which never was the case before. The following example file is called `constant.hpp`. See the warning in line 2.

```c++
#ifndef CONSTANT_HPP
#define CONSTANT_HPP //code/includes outside of area guarded by header guard; consider moving it [llvm-header-guard]

#include

namespace constant
{
//Variables
/**
* @var name
* @brief Programme name.
*/
extern const std::string name;
/**
* @var release
* @brief Release date of the version.
*/
extern const std::string release;
/**
* @var version
* @brief Version.
*/
extern const std::string version;

//Functions
/**
* @brief Creates the programme name in lower case.
* @return Programme name in lower case.
*/
std::string name_lower_case();
} //namespace constant

#endif
```

Start with the clang-tidy `llvm-header-guard` rule and reproduce the warning using the provided `constant.hpp` example, especially the `#define CONSTANT_HPP` line. Trace why this valid guard is treated as code outside the guarded area; done means the example no longer produces the erroneous warning and the behavior is covered by the rule's tests.

Written by the indexing model from the issue text.