ice(): _Alignas() with a negative number

that C code

```c
#include
#include

void greet() {}

struct {} _Alignas(-1) MinusOne;

int main() {
unsigned MinusOne = -1;
}
```

when you try to compile it, you will see an internal compiler error not a regular compiler error

```rust
thread '' (18932) panicked at src/backend/common.rs:1116:9:
alignment must be power of 2
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
ccc: internal error: alignment must be power of 2
```

Instead of handling the invalid code