libstdc++: Consolidate duplicate metaprogramming utilities

libstdc++: Consolidate duplicate metaprogramming utilities

Currently std::variant uses __index_of<T, Types...> to find the first
occurence of a type in a pack, and __exactly_once<T, Types...> to check
that there is no other occurrence.

We can reuse the __find_uniq_type_in_pack<T, Types...>() function for
both tasks, and remove the recursive templates used to implement
__index_of and __exactly_once.

libstdc++-v3/ChangeLog:

	* include/bits/utility.h (__find_uniq_type_in_pack): Move
	definition to here, ...
	* include/std/tuple (__find_uniq_type_in_pack): ... from here.
	* include/std/variant (__detail__variant::__index_of): Remove.
	(__detail::__variant::__exactly_once): Define using
	__find_uniq_type_in_pack instead of __index_of.
	(get<T>, get_if<T>, variant::__index_of): Likewise.