[libc++][ranges][abi-break] Fix `movable_box` overwriting memory of data that...

[libc++][ranges][abi-break] Fix `movable_box` overwriting memory of data that lives in the tail padding (#71314)

fixes #70506 

The detailed problem description is in #70506 

The original proposed fix was to remove `[[no_unique_address]]` except
when `_Tp` is empty.

Edit:
After the discussion in the comments below, the new fix here is to
remove the `[[no_unique_address]]` from `movable_box` in the cases where
we need to add our own assignment operator, which has contains the
problematic `construct_at`