[ADT] Deprecate the redirection from SmallSet to SmallPtrSet (#154891)
This patch deprecates the redirection from SmallSet to SmallPtrSet.
I attempted to deprecate in the usual manner:
template <typename PointeeType, unsigned N>
LLVM_DEPRECATED("...", "...")
class SmallSet<PointeeType*, N> : public SmallPtrSet<PointeeType*, N>
{};
However, the deprecation warning wouldn't fire.
For this reason, I've attached a deprecation message to the default
constructor.