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/backend/nodes/equalfuncs.c

@@ -1211,6 +1211,7 @@ _equalDefineStmt(const DefineStmt *a, const DefineStmt *b)
 	COMPARE_NODE_FIELD(defnames);
 	COMPARE_NODE_FIELD(args);
 	COMPARE_NODE_FIELD(definition);
+	COMPARE_SCALAR_FIELD(if_not_exists);
 
 	return true;
 }