[clang-tools-extra] [clangd] Resolve __builtin_offsetof designator components precisely (PR #194407)

[Aleksandr Platonov via cfe-commits]

https://github.com/ArcsinX commented:

Keeping `VisitOffsetOfExpr()` leads to incorrect behavior when GTD/hover on `__builtin_offsetof` itself, returning field target
E.g.
```cpp
struct S {
  int f;
};
__builtin_o^ffsetof(struct S, f);
```
Hover/GTD at `^` position gives `f` field

https://github.com/llvm/llvm-project/pull/194407