[ADT] Deprecate the redirection from SmallSet to SmallPtrSet (Take 2)

This patch deprecates the SmallSet specialization for pointer types,
which redirects to SmallPtrSet.

My previous attempt in #154891 broke downstream users.  Adding
user-defined constructors with LLVM_DEPRECATED inadvertently caused
the compiler to delete the copy and move assignment operators.

This iteration sidesteps the "Rule of Five" issue entirely by
introducing an intermediate class, DeprecatedSmallSet.  The
deprecation attribute is attached to this new class, and SmallSet
specialization inherits from it.

Author: kazutakahirata

llvm/include/llvm/ADT/SmallSet.h

@@ -268,8 +268,17 @@ class SmallSet {
 
 /// If this set is of pointer values, transparently switch over to using
 /// SmallPtrSet for performance.
+///
+/// We use this middleman class DeprecatedSmallSet so that the deprecation
+/// warning works.  Placing LLVM_DEPRECATED just before SmallSet below won't
+/// work.
+template <typename PointeeType, unsigned N>
+class LLVM_DEPRECATED("Use SmallPtrSet instead", "SmallPtrSet")
+    DeprecatedSmallSet : public SmallPtrSet<PointeeType *, N> {};
+
 template <typename PointeeType, unsigned N>
-class SmallSet<PointeeType*, N> : public SmallPtrSet<PointeeType*, N> {};
+class SmallSet<PointeeType *, N> : public DeprecatedSmallSet<PointeeType *, N> {
+};
 
 /// Equality comparison for SmallSet.
 ///