Return gdbpy_ref from gdb_py_object_from_*longest

This changes gdb_py_object_from_longest and
gdb_py_object_from_ulongest to return a gdbpy_ref rather than a
PyObject*.

gdb/ChangeLog
2018-11-04  Tom Tromey  <tom@tromey.com>

	* python/python-internal.h (gdb_py_object_from_longest)
	(gdb_py_object_from_ulongest): Return gdbpy_ref.
	* python/py-value.c (valpy_int): Update.
	* python/py-utils.c (gdb_py_object_from_longest): Return
	gdbpy_ref.
	(gdb_py_object_from_ulongest): Likewise.
	* python/py-type.c (typy_get_alignof): Update.
	* python/py-linetable.c (ltpy_get_all_source_lines)
	(ltpy_entry_get_line, ltpy_entry_get_pc): Update.
	* python/py-block.c (blpy_get_start, blpy_get_end): Update.