[clang] [Clang][counted_by] Load the count through unions by byte offset (PR #228309)

[Justin Stitt via cfe-commits]

https://github.com/JustinStitt requested changes to this pull request.

im getting a crash when messing around with bitfields:

```c
struct there_be_bitfields_in_these_woods {
  unsigned pad : 13; 
  unsigned count : 13;
  int fam[] __attribute__((counted_by(count)));
};

void set(struct there_be_bitfields_in_these_woods *p, unsigned n) {
  *__builtin_counted_by_ref(p->fam) = n;
}


```

sema should probably reject a bit-field for counted_by? unless we have users for this 😬

https://github.com/llvm/llvm-project/pull/228309