Use bit stride when taking slice of array

Testing with -fgnat-encodings=minimal showed that the Ada code failed
to use the bit stride of an array when taking a slice.  This patch
fixes the oversight.

gdb/ChangeLog
2020-11-04  Tom Tromey  <tromey@adacore.com>

	* ada-lang.c (ada_value_slice_from_ptr): Use bit size.

gdb/testsuite/ChangeLog
2020-11-04  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/array_of_variant.exp: New file.
	* gdb.ada/array_of_variant/p.adb: New file.
	* gdb.ada/array_of_variant/pck.ads: New file.
	* gdb.ada/array_of_variant/pck.adb: New file.