[DWARF] Have HashedNameToDIE store a DataExtractor by value

[labath]

Summary

The DataExtractors are cheap to copy so there is no reason to store them
by reference. Also, in my upcoming indexing refactor I am planning to
remove the apple tables data extractor members from the SymbolFileDWARF
class, so there will not be a DataExtractor with a suitable lifetime to
refer to.

[JDevlieghere]

LGTM

lldb/trunk/source/Plugins/SymbolFile/DWARF/HashedNameToDIE.h

Result AppendHashDataForRegularExpression(
         lldb::offset_t *hash_data_offset_ptr, Pair &pair) const;
 
     size_t FindByName(llvm::StringRef name, DIEInfoArray &die_info_array);
 
     Result GetHashDataForName(llvm::StringRef name,
                               lldb::offset_t *hash_data_offset_ptr,
                               Pair &pair) const override;
 
-    const lldb_private::DWARFDataExtractor &m_data;
-    const lldb_private::DWARFDataExtractor &m_string_table;
+    lldb_private::DWARFDataExtractor m_data;
+    lldb_private::DWARFDataExtractor m_string_table;
     std::string m_name;
   };
 
   static void ExtractDIEArray(const DIEInfoArray &die_info_array,
                               DIEArray &die_offsets);
 
 protected:
   static void ExtractDIEArray(const DIEInfoArray &die_info_array,