Made sure SET DESCRIPTOR accepts all data types including constants.

src/interfaces/ecpg/ChangeLog

src/interfaces/ecpg/ecpglib/data.c

src/interfaces/ecpg/ecpglib/descriptor.c

src/interfaces/ecpg/ecpglib/execute.c

src/interfaces/ecpg/ecpglib/extern.h

src/interfaces/ecpg/include/ecpglib.h

src/interfaces/ecpg/preproc/preproc.y

src/interfaces/ecpg/preproc/variable.c

src/interfaces/ecpg/test/test_desc.pgc