Change ada_which_variant_applies to value API

While debugging an Ada regression, I noticed that all the callers of
ada_which_variant_applies desconstruct a value, only to have it be
reconstructed by this function.

This patch removes this inefficiency in favor of simply passing in the
value directly.

Tested on x86-64 Fedora 30.

gdb/ChangeLog
2020-03-30  Tom Tromey  <tromey@adacore.com>

	* ada-valprint.c (print_variant_part): Update.
	* ada-lang.h (ada_which_variant_applies): Update.
	* ada-lang.c (ada_which_variant_applies): Remove outer_type and
	outer_valaddr parameters; replace with "outer" value parameter.
	(to_fixed_variant_branch_type): Update.