Allow constants in using clauses.

src/interfaces/ecpg/ChangeLog

src/interfaces/ecpg/ecpglib/execute.c

src/interfaces/ecpg/ecpglib/typename.c

src/interfaces/ecpg/include/ecpgtype.h

src/interfaces/ecpg/preproc/preproc.y

src/interfaces/ecpg/preproc/type.c

src/interfaces/ecpg/test/test2.pgc