libstdc++: Add comparison operators to types from Utilities clause

libstdc++: Add comparison operators to types from Utilities clause

Some more C++20 changes from P1614R2, "The Mothership has Landed".

This removes all redundant equality and inequality operators in the
Utilities clause, as they can be synthesized from the remaining equality
operators.

It also removes the single redundant operator in the Localization
clause, because it didn't seem worth doing in a separate commit.

	* include/bits/allocator.h (operator!=): Do not define for C++20.
	* include/bits/locale_classes.h (operator!=): Likewise.
	* include/bits/std_function.h (operator==(nullptr_t, const function&))
	(operator!=(const function&, nullptr_t))
	(operator!=(nullptr_t, const function&)): Likewise.
	* include/ext/bitmap_allocator.h (operator!=): Likewise.
	* include/ext/debug_allocator.h (operator!=): Likewise.
	* include/ext/extptr_allocator.h (operator!=): Likewise.
	* include/ext/malloc_allocator.h (operator!=): Likewise.
	* include/ext/mt_allocator.h (operator!=): Likewise.
	* include/ext/new_allocator.h (operator!=): Likewise.
	* include/ext/pool_allocator.h (operator!=): Likewise.
	* include/ext/throw_allocator.h (operator!=): Likewise.
	* include/std/bitset (bitset::operator!=): Likewise.
	* include/std/memory_resource (operator!=): Likewise.
	* include/std/scoped_allocator (operator!=): Likewise.