Second phase of committing Rod Taylor's pg_depend/pg_constraint patch.

pg_relcheck is gone; CHECK, UNIQUE, PRIMARY KEY, and FOREIGN KEY
constraints all have real live entries in pg_constraint.  pg_depend
exists, and RESTRICT/CASCADE options work on most kinds of DROP;
however, pg_depend is not yet very well populated with dependencies.
(Most of the ones that are present at this point just replace formerly
hardwired associations, such as the implicit drop of a relation's pg_type
entry when the relation is dropped.)  Need to add more logic to create
dependency entries, improve pg_dump to dump constraints in place of
indexes and triggers, and add some regression tests.

src/include/commands/proclang.h

@@ -13,5 +13,6 @@
 
 extern void CreateProceduralLanguage(CreatePLangStmt *stmt);
 extern void DropProceduralLanguage(DropPLangStmt *stmt);
+extern void DropProceduralLanguageById(Oid langOid);
 
 #endif   /* PROCLANG_H */