Fix accessing a method's fields from Python

Fix accessing a method's fields from Python

Considering this example:

struct C
{
  int func() { return 1; }
} c;
int main()
{
  return c.func();
}

Accessing the fields of C::func, when requesting the function by its
type, works:

(gdb) py print(gdb.parse_and_eval('C::func').type.fields()[0].type)
C * const

But when trying to do the same via a class instance, it fails:

(gdb) py print(gdb.parse_and_eval('c')['func'].type.fields()[0].type)
Traceback (most recent call last):
  File "<string>", line 1, in <module>
TypeError: Type is not a structure, union, enum, or function type.
Error while executing Python code.

The difference is that in the former the function type is TYPE_CODE_FUNC:

(gdb) py print(gdb.parse_and_eval('C::func').type.code == gdb.TYPE_CODE_FUNC)
True

And in the latter the function type is TYPE_CODE_METHOD:

(gdb) py print(gdb.parse_and_eval('c')['func'].type.code == gdb.TYPE_CODE_METHOD)
True

So this adds the functionality for TYPE_CODE_METHOD as well.

gdb/ChangeLog:

2020-12-18  Hannes Domani  <ssbssa@yahoo.de>

	* python/py-type.c (typy_get_composite): Add TYPE_CODE_METHOD.

gdb/testsuite/ChangeLog:

2020-12-18  Hannes Domani  <ssbssa@yahoo.de>

	* gdb.python/py-type.exp: Add tests for TYPE_CODE_METHOD.