gdb/python: add function () method to gdb.Type object

gdb/python: add function () method to gdb.Type object

This commit adds a new method to Python type objects that returns
possibly new function type returning that type. Parameter types can
be specified too.

This will be useful later to create types for function symbols created
using Python extension code.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>