Fix gdb.ada/vla.exp

PR ada/24539 concerns a test failure in gdb.ada/vla.exp.

The problem here is that different versions of Gnat emit the
structure's fields in different orders -- with the order currently
failing actually being the correct one.

Joel pointed out that this can be fixed by simply adding the
No_Component_Reordering pragma to the type in question, which is what
this patch does.

I've reported a Gnat compiler bug internally in hopes of getting the
underlying problem fixed.

gdb/testsuite/ChangeLog
2019-06-14  Tom Tromey  <tromey@adacore.com>

	PR ada/24539:
	* gdb.ada/vla.exp: Update tests.
	* gdb.ada/vla/vla.adb (Record_Type): Use No_Component_Reordering
	pragma.