[llvm] [ADT] Deprecate the redirection from SmallSet to SmallPtrSet (PR #154891)

[Yanzuo Liu via llvm-commits]

zwuis wrote:

```cpp
[[deprecated]] struct S1 {} var1; // `var1` is deprecated
struct [[deprecated]] S2 {} var2; // `S2` is deprecated
```

https://github.com/llvm/llvm-project/pull/154891