libstdc++: Guard uses of is_pointer_interconvertible_v [PR114891]

This type trait isn't supported by Clang 18. It's only used in static
assertions, so they can just be omitted if the trait isn't available.

libstdc++-v3/ChangeLog:

	PR libstdc++/114891
	* include/std/generator: Check feature test macro before using
	is_pointer_interconvertible_v.