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/copyfuncs.c

@@ -3105,6 +3105,7 @@ _copyDefineStmt(const DefineStmt *from)
 	COPY_NODE_FIELD(defnames);
 	COPY_NODE_FIELD(args);
 	COPY_NODE_FIELD(definition);
+	COPY_SCALAR_FIELD(if_not_exists);
 
 	return newnode;
 }