Fix accessing a function's fields (parameters) from Python (PR 18073)

Fix accessing a function's fields (parameters) from Python (PR 18073)

Since 7.4, gdb doesn't allow calling .fields() on a function type, even
though the documentation states it should return a list corresponding to
the function's parameters.  This patch restores the intended behaviour
and adds a test for it.

Reg-tested on Arch Linux x86-64.

gdb/ChangeLog:

	PR python/18073
	* python/py-type.c (typy_get_composite): Allow returning a
	function type.

gdb/testsuite/ChangeLog:

	PR python/18073
	* gdb.python/py-type.c (C::a_method): New.
	(C::a_const_method): New.
	(C::a_static_method): New.
	(a_function): New.
	* gdb.python/py-type.exp (test_fields): Test getting fields
	from function and method.