[C2x] reject type definitions in offsetof

https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2350.htm made very
clear that it is an UB having type definitions with in offsetof. After
this patch clang will reject any type definitions in __builtin_offsetof.

Fixes https://github.com/llvm/llvm-project/issues/57065

```
local/offsetof.c:10:38: error: 'struct S' cannot be defined in '__builtin_offsetof'
    return __builtin_offsetof(struct S{ int a, b;}, a);
                                     ^
```

Reviewed By: aaron.ballman, #clang-language-wg

Differential Revision: https://reviews.llvm.org/D133574