[clang] Extension: allow recursive macros (PR #65851)

[via cfe-commits]

kelbon wrote:

Version with `__THIS_MACRO__` instead of new preprocessor directive is ready, so actual examples are like

```C++
#define reverse(head, ...) __VA_OPT__(__THIS_MACRO__(__VA_ARGS__) , ) head
```
`__THIS_MACRO__` behaves exactly as macro name(`reverse` in this case), but its allowed to be expanded recursively

@cor3ntin i dont understand what github wants from me (changes requested)

https://github.com/llvm/llvm-project/pull/65851