[llvm] [ADT] Deprecate the redirection from SmallSet to SmallPtrSet (Take 2) (PR #155078)

[Jinsong Ji via llvm-commits]

jsji wrote:

Any downstream files trying to use headers that has #include <SmallSet.h> will get warnings, there is no use of SmallSet at all.



```
llvm-project/build# cat t.cpp
#include "llvm/CodeGen/MachineDominators.h"
llvm-project/build# /usr/bin/c++ t.cpp -I llvm-project/llvm/include -Illvm-project/build/include -c

In file included from llvm-project/llvm/include/llvm/CodeGen/MachineDominators.h:17,
                 from t.cpp:1:
llvm-project/llvm/include/llvm/ADT/SmallSet.h:280:43: warning: ‘template<class PointerType, unsigned int N> class llvm::DeprecatedSmallSet’ is deprecated: Use SmallPtrSet instead [-Wdeprecated-declarations]
  280 | class SmallSet<PointeeType *, N> : public DeprecatedSmallSet<PointeeType *, N> {
      |                                           ^~~~~~~~~~~~~~~~~~
llvm-project/llvm/include/llvm/ADT/SmallSet.h:277:5: note: declared here
  277 |     DeprecatedSmallSet : public SmallPtrSet<PointerType, N> {};
      |     ^~~~~~~~~~~~~~~~~~

```

https://github.com/llvm/llvm-project/pull/155078