Select variant field when printing variant

Select variant field when printing variant

When I updated the Ada variant-printing code to be value-based, I
neglected a couple of issues.  First, print_variant_part must first
extract the variant field before finding the active component; second,
print_field_values should pass in the field value as the outer value
when recursing.

This patch fixes both of these issues.

gdb/ChangeLog
2020-04-06  Tom Tromey  <tromey@adacore.com>

	* ada-valprint.c (print_variant_part): Extract the variant field.
	(print_field_values): Use the field as the outer value when
	recursing.

gdb/testsuite/ChangeLog
2020-04-06  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/variant-record/proc.adb: New file.
	* gdb.ada/variant-record/value.adb: New file.
	* gdb.ada/variant-record/value.s: New file.
	* gdb.ada/variant-record.exp: New file.