Fix Ada val_print removal regression

Fix Ada val_print removal regression

The removal of val_print caused a regression in the Ada code.  In one
scenario, a variant type would not be properly printed, because the
address of a component was lost.  This patch fixes the bug by changing
this API to be value-based.  This is cleaner and fixes the bug as a
side effect.

gdb/ChangeLog
2020-03-20  Tom Tromey  <tromey@adacore.com>

	* ada-valprint.c (print_variant_part): Remove parameters; switch
	to value-based API.
	(print_field_values): Likewise.
	(ada_val_print_struct_union): Likewise.
	(ada_value_print_1): Update.

gdb/testsuite/ChangeLog
2020-03-20  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/sub_variant/subv.adb: New file.
	* gdb.ada/sub_variant.exp: New file.