Fix crash with GNAT minimal encodings

Fix crash with GNAT minimal encodings

Running the AdaCore internal test suite with -fgnat-encodings=minimal
found a gdb crash.  The bug is that GDB ends up with a typedef in
ada_index_type, resulting in a NULL dereference.

This crash can be reproduced using GCC 11 with the included test case.

Tested on x86-64 Fedora 32.  Because this is Ada-specific, and was
already reviewed by Joel, I am going to check it in.

2021-04-30  Tom Tromey  <tromey@adacore.com>

	* ada-lang.c (ada_index_type): Use ada_check_typedef.

gdb/testsuite/ChangeLog
2021-04-30  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/enum_idx_packed/pck.ads (My_Enum, My_Array_Type)
	(Confused_Array): New types.
	* gdb.ada/enum_idx_packed/foo.adb (Confused_Array): New variable.
	* gdb.ada/enum_idx_packed.exp: Add new tests.