Add an objfile getter to gdb.Type

This allows users of the Python API to find the objfile where a type
was defined.

gdb/ChangeLog:

gdb/ChangeLog
2019-06-04  Christian Biesinger  <cbiesinger@google.com>

	Add objfile property to gdb.Type.
	* gdb/NEWS: Mention Python API addition.
	* gdb/python/py-type.c (typy_get_objfile): New method.

gdb/doc/ChangeLog
2019-06-04  Christian Biesinger  <cbiesinger@google.com>

	* gdb/doc/python.texi: Document new gdb.Type.objfile property.

gdb/testsuite/ChangeLog
2019-06-04  Christian Biesinger  <cbiesinger@google.com>

	* gdb/testsuite/gdb.python/py-type.exp: Test for new
	  gdb.Type.objfile property.