Add DOMAIN check constraints.

doc/src/sgml/ref/create_domain.sgml

src/backend/catalog/heap.c

src/backend/catalog/pg_constraint.c

src/backend/commands/tablecmds.c

src/backend/commands/typecmds.c

src/backend/executor/execQual.c

src/backend/nodes/copyfuncs.c

src/backend/nodes/equalfuncs.c

src/backend/nodes/outfuncs.c

src/backend/nodes/readfuncs.c

src/backend/optimizer/util/clauses.c

src/backend/parser/analyze.c

src/backend/parser/gram.y

src/backend/parser/keywords.c

src/backend/parser/parse_clause.c

src/backend/parser/parse_coerce.c

src/backend/parser/parse_expr.c

src/backend/parser/parse_node.c

src/backend/parser/parse_target.c

src/backend/utils/adt/ruleutils.c