Use the unified section list when generating a symbol table

[labath]

Summary

Symbol table generation code was failing to take into account the debug symbols because
the object file was looking only into its own section list when doing the generation, even though
the debug symbols from another object file were correctly detected and loaded by the
SymbolVendor. This changes the code to use the unified section list, which fixes this problem.

[clayborg]

Looks good.

source/Plugins/ObjectFile/ELF/ObjectFileELF.cpp

ObjectFileELF::GetSymtab()
     // We always want to use the main object file so we (hopefully) only have one cached copy
     // of our symtab, dynamic sections, etc.
     ObjectFile *module_obj_file = module_sp->GetObjectFile();
     if (module_obj_file && module_obj_file != this)
         return module_obj_file->GetSymtab();
 
     if (m_symtab_ap.get() == NULL)
     {
-        SectionList *section_list = GetSectionList();
+        SectionList *section_list = module_sp->GetSectionList();
         if (!section_list)
             return NULL;
 
         uint64_t symbol_id = 0;
         lldb_private::Mutex::Locker locker(module_sp->GetMutex());
 
         m_symtab_ap.reset(new Symtab(this));