Closes#132561.
This is a check that rewrites `#if`s and `#elif`s like so:
```cpp
#if defined(MEOW) // -> #ifdef MEOW
#if !defined(MEOW) // -> #ifndef MEOW
```
And, since C23 and C++23:
```cpp
#elif defined(MEOW) // -> #elifdef MEOW
#elif !defined(MEOW) // -> #elifndef MEOW
```
