Add support for renaming domain constraints

doc/src/sgml/ref/alter_domain.sgml

src/backend/catalog/objectaddress.c

src/backend/catalog/pg_constraint.c

src/backend/commands/tablecmds.c

src/backend/commands/typecmds.c

src/backend/parser/gram.y

src/backend/parser/parse_utilcmd.c

src/include/catalog/pg_constraint.h

src/include/commands/typecmds.h

src/test/regress/expected/domain.out

src/test/regress/sql/domain.sql