Do not expose stub types to Python

Do not expose stub types to Python

dwarf2read.c will create stub types for Ada "Taft Amendment" types.
These stub types can currently be exposed to Python code, where they
show up as TYPE_CODE_VOID types (but that, mysteriously, can sometimes
be used in other ways).

While it's possible to work with such types by using strip_typedefs,
this seemed unpleasant to me.  This patch takes another approach
instead, which is to try not to expose stub types to Python users.

gdb/ChangeLog
2019-09-26  Tom Tromey  <tromey@adacore.com>

	* python/py-type.c (type_to_type_object): Call check_typedef
	for stub types.

gdb/testsuite/ChangeLog
2019-09-26  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/py_taft.exp: New file.
	* gdb.ada/py_taft/main.adb: New file.
	* gdb.ada/py_taft/pkg.adb: New file.
	* gdb.ada/py_taft/pkg.ads: New file.