[lldb] Replace SmallSet with SmallPtrSet (NFC) (llvm#154366)

This patch replaces SmallSet<T *, N> with SmallPtrSet<T *, N>.  Note
that SmallSet.h "redirects" SmallSet to SmallPtrSet for pointer
element types:

  template <typename PointeeType, unsigned N>
class SmallSet<PointeeType*, N> : public SmallPtrSet<PointeeType*, N>
{};

We only have 10 instances that rely on this "redirection". Since the
redirection doesn't improve readability, this patch replaces SmallSet
with SmallPtrSet for pointer element types.

I'm planning to remove the redirection eventually.

Author: kazutakahirata

lldb/source/Plugins/SymbolFile/DWARF/DWARFDIE.cpp

@@ -44,7 +44,7 @@ class ElaboratingDIEIterator
   // Container sizes are optimized for the case of following DW_AT_specification
   // and DW_AT_abstract_origin just once.
   llvm::SmallVector<DWARFDIE, 2> m_worklist;
-  llvm::SmallSet<DWARFDebugInfoEntry *, 3> m_seen;
+  llvm::SmallPtrSet<DWARFDebugInfoEntry *, 3> m_seen;
 
   void Next() {
     assert(!m_worklist.empty() && "Incrementing end iterator?");