[lldb] [nfc] Simplify user_id_t -> size_t

As discussed in https://reviews.llvm.org/D73206#1871895> simplifying
usage of `user_id_t`.

There is even written:
  // The compile unit ID is the index of the DWARF unit.
  DWARFUnit *dwarf_cu = info->GetUnitAtIndex(comp_unit->GetID());

Differential Revision: https://reviews.llvm.org/D74670

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

@@ -109,7 +109,7 @@ size_t DWARFDebugInfo::GetNumUnits() {
   return m_units.size();
 }
 
-DWARFUnit *DWARFDebugInfo::GetUnitAtIndex(user_id_t idx) {
+DWARFUnit *DWARFDebugInfo::GetUnitAtIndex(size_t idx) {
   DWARFUnit *cu = nullptr;
   if (idx < GetNumUnits())
     cu = m_units[idx].get();

lldb/source/Plugins/SymbolFile/DWARF/DWARFDebugInfo.h

@@ -35,7 +35,7 @@ class DWARFDebugInfo {
                           lldb_private::DWARFContext &context);
 
   size_t GetNumUnits();
-  DWARFUnit *GetUnitAtIndex(lldb::user_id_t idx);
+  DWARFUnit *GetUnitAtIndex(size_t idx);
   DWARFUnit *GetUnitAtOffset(DIERef::Section section, dw_offset_t cu_offset,
                              uint32_t *idx_ptr = nullptr);
   DWARFUnit *GetUnitContainingDIEOffset(DIERef::Section section,