Update Ada ptype support for dynamic types

Update Ada ptype support for dynamic types

The DWARF reader was updated to handle variant parts and some other
selected features for Ada; but the Ada "ptype" code was not touched.
This patch changes the Ada ptype code to handle the new types
properly.

Test cases for this and for some of the other code in this series are
in a separate patch.

gdb/ChangeLog
2020-04-24  Tom Tromey  <tromey@adacore.com>

	* ada-typeprint.c (print_choices, print_variant_part)
	(print_record_field_types_dynamic): New functions.
	(print_record_field_types): Use print_record_field_types_dynamic.