libstdc++: Replace uses of EBO with [[no_unique_address]]

libstdc++: Replace uses of EBO with [[no_unique_address]]

Clang 9 added support for [[__no_unique_address__]] and we don't support
Intel icc any longer, so we can remove the code in <tuple> that works
around the absence of that attribute. We can also address a FIXME in
<bits/shared_ptr_base.h> and replace uses of EBO with the attribute.

libstdc++-v3/ChangeLog:

	* include/bits/shared_ptr_base.h (_Sp_ebo_helper): Simplify by
	using [[__no_unique_address__]] instead of EBO. Use the
	attribute unconditionally for the unstable ABI.
	(_Sp_counted_deleter::_Impl): Adjust uses of _Sp_ebo_helper.
	(_Sp_counted_ptr_inplace::_Impl): Likewise.
	* include/std/tuple (_Head_base): Remove implementation for
	compilers that don't support [[__no_unique_address__]]. Use the
	attribute unconditionally for the unstable ABI.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>