Libstdc++: Replace uses of EBO with [[no_unique_address]]
The libstdc++ now uses the `[[no_unique_address]]` attribute instead of empty base optimization (EBO) to reduce the size of class layouts.
The libstdc++ now uses the [[no_unique_address]] attribute instead of empty base optimization (EBO) in <tuple> and <bits/shared_ptr_base.h>. This modernizes the code and simplifies the implementation on compilers that support the attribute, which includes Clang (since v9) and GCC itself. This change reduces the size of class layouts by allowing empty base classes to be stored in the same memory location as other members.
In Details
This commit replaces EBO with [[no_unique_address]] in std::tuple and _Sp_counted_deleter within <bits/shared_ptr_base.h>. EBO was previously used to optimize the layout of classes containing empty base classes. [[no_unique_address]] provides a more direct way to achieve the same goal. This change simplifies the code by removing workarounds for older compilers. The use of the attribute is now unconditional for the unstable ABI. Toolchain devs should note the changes to _Sp_ebo_helper and _Head_base.
For Context
In C++, Empty Base Optimization (EBO) is a compiler optimization technique that allows empty base classes to occupy zero space in a derived class. [[no_unique_address]] is a C++17 attribute that provides a standard way to achieve the same effect, allowing empty members to be stored in the same memory location. This commit updates the standard library to use this attribute, potentially leading to smaller and more efficient data structures.