Give more informative error message for by-reference types

Recent compilers enforce more strictly the RM C.6(18) clause, which says
that volatile record types are by-reference types.  This changes the typical
error message now given in these cases.

gcc/ada/
	* gcc-interface/decl.c (gnat_to_gnu_entity) <is_type>: Declare new
	constant.  Adjust error message issued by validate_size in the case
	of by-reference types.
	(validate_size): Always use the error strings passed by the caller.