Add make_date() and make_time() functions.

doc/src/sgml/func.sgml

src/backend/utils/adt/date.c

src/backend/utils/adt/datetime.c

src/include/catalog/catversion.h

src/include/catalog/pg_proc.h

src/include/utils/date.h

src/include/utils/datetime.h

src/test/regress/expected/date.out

src/test/regress/sql/date.sql