[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.

Author: kazutakahirata

llvm/include/llvm/ADT/SmallSet.h

@@ -269,7 +269,28 @@ class SmallSet {
 /// If this set is of pointer values, transparently switch over to using
 /// SmallPtrSet for performance.
 template <typename PointeeType, unsigned N>
-class SmallSet<PointeeType*, N> : public SmallPtrSet<PointeeType*, N> {};
+class SmallSet<PointeeType *, N> : public SmallPtrSet<PointeeType *, N> {
+  using Base = SmallPtrSet<PointeeType *, N>;
+
+public:
+  // LLVM_DEPRECATED placed between "template" and "class" above won't work for
+  // some reason.  Put a deprecation message on constructors instead.
+  LLVM_DEPRECATED("Use SmallPtrSet instead", "SmallPtrSet")
+  SmallSet() = default;
+  LLVM_DEPRECATED("Use SmallPtrSet instead", "SmallPtrSet")
+  SmallSet(const SmallSet &) = default;
+  LLVM_DEPRECATED("Use SmallPtrSet instead", "SmallPtrSet")
+  SmallSet(SmallSet &&) = default;
+  template <typename IterT>
+  LLVM_DEPRECATED("Use SmallPtrSet instead", "SmallPtrSet")
+  SmallSet(IterT Begin, IterT End) : Base(Begin, End) {}
+  template <typename Range>
+  LLVM_DEPRECATED("Use SmallPtrSet instead", "SmallPtrSet")
+  SmallSet(llvm::from_range_t, Range &&R)
+      : Base(llvm::from_range, std::move(R)) {}
+  LLVM_DEPRECATED("Use SmallPtrSet instead", "SmallPtrSet")
+  SmallSet(std::initializer_list<PointeeType *> L) : Base(L) {}
+};
 
 /// Equality comparison for SmallSet.
 ///