Add CREATE COLLATION IF NOT EXISTS clause

The core of the functionality was already implemented when
pg_import_system_collations was added.  This just exposes it as an
option in the SQL command.

src/include/nodes/parsenodes.h

@@ -2380,6 +2380,7 @@ typedef struct DefineStmt
 	List	   *defnames;		/* qualified name (list of Value strings) */
 	List	   *args;			/* a list of TypeName (if needed) */
 	List	   *definition;		/* a list of DefElem */
+	bool		if_not_exists;	/* just do nothing if it already exists? */
 } DefineStmt;
 
 /* ----------------------