Move pg_constraint.h function declarations to new file pg_constraint_fn.h.

A pending patch requires exporting a function returning Bitmapset from
catalog/pg_constraint.c.  As things stand, that would mean including
nodes/bitmapset.h in pg_constraint.h, which might be hazardous for the
client-side includability of that header.  It's not entirely clear whether
any client-side code needs to include pg_constraint.h, but it seems prudent
to assume that there is some such code somewhere.  Therefore, split off the
function definitions into a new file pg_constraint_fn.h, similarly to what
we've done for some other catalog header files.

src/backend/commands/tablecmds.c

@@ -32,6 +32,7 @@
 #include "catalog/pg_am.h"
 #include "catalog/pg_collation.h"
 #include "catalog/pg_constraint.h"
+#include "catalog/pg_constraint_fn.h"
 #include "catalog/pg_depend.h"
 #include "catalog/pg_foreign_table.h"
 #include "catalog/pg_inherits.h"

src/backend/commands/trigger.c

@@ -23,6 +23,7 @@
 #include "catalog/indexing.h"
 #include "catalog/objectaccess.h"
 #include "catalog/pg_constraint.h"
+#include "catalog/pg_constraint_fn.h"
 #include "catalog/pg_proc.h"
 #include "catalog/pg_trigger.h"
 #include "catalog/pg_type.h"

src/backend/commands/typecmds.c

@@ -41,6 +41,7 @@
 #include "catalog/pg_authid.h"
 #include "catalog/pg_collation.h"
 #include "catalog/pg_constraint.h"
+#include "catalog/pg_constraint_fn.h"
 #include "catalog/pg_depend.h"
 #include "catalog/pg_enum.h"
 #include "catalog/pg_language.h"