[clang] [clang][Sema] Emit warnings on incorrect AVR interrupt/signal handlers (PR #125328)

[via cfe-commits]

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aykevl wrote:

It might be worth adding a check for something like this?

```c
__attribute__((signal)) void foog() {} // expected-warning {{'signal' attribute only applies to functions that have no parameters}}
```

(This is because C, at least in the past, treated no parameters as a variadic function).

https://github.com/llvm/llvm-project/pull/125328