Add DIERef::operator==

This was needed for the Rust plugin

Author: tromey

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

@@ -44,6 +44,8 @@ class DIERef {
 
   dw_offset_t die_offset() const { return m_die_offset; }
 
+  bool operator==(const DIERef &ref) const { return die_offset == ref.die_offset; }
+
 private:
   uint32_t m_dwo_num : 30;
   uint32_t m_dwo_num_valid : 1;