[llvm] [ADT] Fix redirection of SmallSet to SmallPtrSet (PR #155117)

[Ben Langmuir via llvm-commits]

benlangmuir wrote:

I spent a while trying to write a test or static_assert for this but couldn't find one that didn't emit a warning.  The usual `LLVM_SUPPRESS_DEPRECATED_DECLARATIONS_PUSH` didn't work, because the warning location is on the `SmallSet : public DeprecatedSmallSet` line and we specifically want that to emit a warning if it gets used.  Let me know if you have an idea.

https://github.com/llvm/llvm-project/pull/155117