libstdc++: Reduce header dependencies in and on <memory>

libstdc++: Reduce header dependencies in and on <memory>

By moving std::make_obj_using_allocator and the related "utility
functions for uses-allocator construction" to a new header, we can avoid
including the whole of <memory> in <scoped_allocator> and
<memory_resource>.

In order to simplify the implementation of those utility functions they
now use concepts unconditionally. They are no longer defined if
__cpp_concepts is not defined. To simplify the code that uses those
functions I've introduced a __cpp_lib_make_obj_using_allocator feature
test macro (not specified in the standard, which might be an oversight).
That allows the code in <memory_resource> and <scoped_allocator> to
check the feature test macro to decide whether to use the new utilities,
or fall back to the C++17 code.

At the same time, this reshuffles some of the headers included by
<memory> so that they are (mostly?) self-contained. It should no longer
be necessary to include other headers before <bits/shared_ptr.h> when
other parts of the library want to use std::shared_ptr without including
the whole of <memory>.

libstdc++-v3/ChangeLog:

	* include/Makefile.am: Add new header.
	* include/Makefile.in: Regenerate.
	* include/bits/shared_ptr.h: Include <iosfwd>.
	* include/bits/shared_ptr_base.h: Include required headers here
	directly, instead of in <memory>.
	* include/bits/uses_allocator_args.h: New file. Move utility
	functions for uses-allocator construction from <memory> to here.
	Only define the utility functions when concepts are available.
	(__cpp_lib_make_obj_using_allocator): Define non-standard
	feature test macro.
	* include/std/condition_variable: Remove unused headers.
	* include/std/future: Likewise.
	* include/std/memory: Remove headers that are not needed
	directly, and are now inclkuded where they're needed. Include
	new <bits/uses_allocator_args.h> header.
	* include/std/memory_resource: Include only the necessary
	headers. Use new feature test macro to detect support for the
	utility functions.
	* include/std/scoped_allocator: Likewise.
	* include/std/version (__cpp_lib_make_obj_using_allocator):
	Define.