libstdc++: Only declare feature test macros in standard headers

libstdc++: Only declare feature test macros in standard headers

This change moves the definitions of feature test macros (or strictly
speaking, the requests for <bits/version.h> to define them) so that only
standard headers define them. For example, <bits/shared_ptr.h> will no
longer define macros related to std::shared_ptr, only <memory> and
<version> will define them. This means that __cpp_lib_shared_ptr_arrays
will not be defined by <future> or by other headers that include
<bits/shared_ptr.h>. It will only be defined when <memory> has been
included. This will discourage users from relying on transitive
includes.

As a result, internal headers that need to query the macros should use
the internal macros like __glibcxx_shared_ptr_arrays instead of
__cpp_lib_shared_ptr_arrays, as those internal macros are defined by the
internal headers after icluding <bits/version.h>. There are some
exceptions to this rule, because __cpp_lib_is_constant_evaluated is
defined by bits/c++config.h and so is available everywhere, and
__cpp_lib_three_way_comparison is defined by <compare> which several
headers are explicitly specified to include, so its macro is guaranteed
to be usable too.

N.B. not many internal headers actually need an explicit include of
<bits/version.h>, because most of them include <type_traits> and so get
all the __glibcxx_foo internal macros from there.

libstdc++-v3/ChangeLog:

	* include/bits/algorithmfwd.h: Do not define standard feature
	test macro here.
	* include/bits/align.h: Likewise. Test internal macros instead
	of standard macros.
	* include/bits/alloc_traits.h: Likewise.
	* include/bits/allocator.h: Likewise.
	* include/bits/atomic_base.h: Likewise.
	* include/bits/atomic_timed_wait.h: Likewise.
	* include/bits/atomic_wait.h: Likewise.
	* include/bits/basic_string.h: Likewise.
	* include/bits/basic_string.tcc: Likewise.
	* include/bits/char_traits.h: Likewise.
	* include/bits/chrono.h: Likewise.
	* include/bits/cow_string.h: Likewise.
	* include/bits/forward_list.h: Likewise.
	* include/bits/hashtable.h: Likewise.
	* include/bits/ios_base.h: Likewise.
	* include/bits/memory_resource.h: Likewise.
	* include/bits/move.h: Likewise.
	* include/bits/move_only_function.h: Likewise.
	* include/bits/node_handle.h: Likewise.
	* include/bits/ptr_traits.h: Likewise.
	* include/bits/range_access.h: Likewise.
	* include/bits/ranges_algo.h: Likewise.
	* include/bits/ranges_cmp.h: Likewise.
	* include/bits/ranges_util.h: Likewise.
	* include/bits/semaphore_base.h: Likewise.
	* include/bits/shared_ptr.h: Likewise.
	* include/bits/shared_ptr_atomic.h: Likewise.
	* include/bits/shared_ptr_base.h: Likewise.
	* include/bits/stl_algo.h: Likewise.
	* include/bits/stl_algobase.h: Likewise.
	* include/bits/stl_function.h: Likewise.
	* include/bits/stl_iterator.h: Likewise.
	* include/bits/stl_list.h: Likewise.
	* include/bits/stl_map.h: Likewise.
	* include/bits/stl_pair.h: Likewise.
	* include/bits/stl_queue.h: Likewise.
	* include/bits/stl_stack.h: Likewise.
	* include/bits/stl_tree.h: Likewise.
	* include/bits/stl_uninitialized.h: Likewise.
	* include/bits/stl_vector.h: Likewise.
	* include/bits/unique_ptr.h: Likewise.
	* include/bits/unordered_map.h: Likewise.
	* include/bits/uses_allocator_args.h: Likewise.
	* include/bits/utility.h: Likewise.
	* include/bits/erase_if.h: Add comment.
	* include/std/algorithm: Define standard feature test macros
	here.
	* include/std/atomic: Likewise.
	* include/std/array: Likewise.
	* include/std/chrono: Likewise.
	* include/std/condition_variable: Likewise.
	* include/std/deque: Likewise.
	* include/std/format: Likewise.
	* include/std/functional: Likewise.
	* include/std/forward_list: Likewise.
	* include/std/ios: Likewise.
	* include/std/iterator: Likewise.
	* include/std/list: Likewise.
	* include/std/map: Likewise.
	* include/std/memory: Likewise.
	* include/std/numeric: Likewise.
	* include/std/queue: Likewise.
	* include/std/ranges: Likewise.
	* include/std/regex: Likewise.
	* include/std/set: Likewise.
	* include/std/stack: Likewise.
	* include/std/stop_token: Likewise.
	* include/std/string: Likewise.
	* include/std/string_view:
	* include/std/tuple: Likewise.
	* include/std/unordered_map:
	* include/std/unordered_set:
	* include/std/utility: Likewise.
	* include/std/vector: Likewise.
	* include/std/scoped_allocator: Query internal macros instead of
	standard macros.