[clang] [clang][bytecode] Fix unknown size arrays crash in clang bytecode (PR #160015)

[Timm Baeder via cfe-commits]

tbaederr wrote:

This works:
```c++

extern const char s[];
void foo(char *x)
{
    char *r;
    __builtin_strcpy(r, s);
}

```

https://github.com/llvm/llvm-project/pull/160015