1. Record the de-referenced expression to newly added field ref_expr
   of strcut asn1p_ref_t.
2. In function asn1c_make_identifier(), not only check whehter there
   is name clash occurred for this asn1p_expr_t but also the referenced
   expression to determine whether an additional module name to be added.
3. Change signature of some functions and variables to eliminate warning
   messages when const type pointer, i.e. const asn1p_ref_t *, variables
   versus, non-const * type pointer, i.e. asn1p_ref_t *, varibles are
   used. E.g.:
   warning: assignment discards ‘const’ qualifier from pointer target type