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

[Ben Langmuir via llvm-commits]

benlangmuir wrote:

This code is turning `SmallSet<T *>` into `SmallPtrSet<T **>` (extra pointer level), which results in build errors if it gets used.  The issue is `DeprecatedSmallSet` is a template, not a specialization, so its `PointeeType` is the pointer not the pointee.

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