libstdc++: Add __is_in_place_index_v helper and use it in <variant>

libstdc++: Add __is_in_place_index_v helper and use it in <variant>

We already have __is_in_place_type_v for in_place_type_t so adding an
equivalent for in_place_index_t allows us avoid a class template
instantiation for the __not_in_place_tag constraint on the most
commonly-used std::variant::variant(T&&) constructor.

For in_place_type_t we also have a __is_in_place_type class template
defined in terms of the variable template, but that isn't actually used
anywhere. I'm not adding an equivalent for the new variable template,
because that wouldn't be used either.

For GCC 15 we should remove the unused __is_in_place_tag and
__is_in_place_type class templates.

libstdc++-v3/ChangeLog:

	* include/bits/utility.h (__is_in_place_index_v): New variable
	template.
	* include/std/variant (__not_in_place_tag): Define in terms of
	variable templates not a class template.