[clang] [Clang] Implement P2809: Trivial infinite loops are not Undefined Behavior (PR #90066)

[via cfe-commits]

yronglin wrote:

I'd like to add the test case from the paper:

```
#include <iostream>

int main() {
  while (true)
    ; 
}

void unreachable() {
  std::cout << "Hello world!" << std::endl;
}
```

https://github.com/llvm/llvm-project/pull/90066