[NFC][libc++] Removes sliceExpr friend.

[Mordante]

Summary

The class is never defined. This was discovered while validating modules
in libc++.

[philnik]

I think Eric wanted to approve, not block this.

[EricWF]

Sorry, I meant to comment that this is likely a misspelling of __slice_expr. Though I don't know if it's needed, and the test coverage here is really poor. But I would rather change it to what the author likely intended it to be.

[EricWF]

If we think there's no need for the friend declaration, then lets explain why we think it's unneeded in the commit message for archeology.

Because I suspect the addition of the friend declaration for __slice_expr without needing it points to a bit of unimplemented code.
But I don't think anybody want's to work on improving <valarray>?

[Mordante]

In D148357#4271209, @EricWF wrote:

If we think there's no need for the friend declaration, then lets explain why we think it's unneeded in the commit message for archeology.

Because I suspect the addition of the friend declaration for __slice_expr without needing it points to a bit of unimplemented code.

As discussed on Discord I'll revert and reland with a better commit message.

But I don't think anybody want's to work on improving <valarray>?

When I read about valarray the first time it sounded very interesting. Maybe when C++ has SIMD support it might be interesting to see whether that can be used for valarray. (I have hopes SIMD will be available in C++26.)

[Mordante]

Relanded 2d7bb01840067d2b9f6b02bd633ca914d513544b

libcxx/include/valarray

private:
     _LIBCPP_INLINE_VISIBILITY
     slice_array(const slice& __sl, const valarray<value_type>& __v)
         : __vp_(const_cast<value_type*>(__v.__begin_ + __sl.start())),
           __size_(__sl.size()),
           __stride_(__sl.stride())
         {}
 
     template <class> friend class valarray;
-    template <class> friend class sliceExpr;
 };
 
 template <class _Tp>
 inline
 const slice_array<_Tp>&
 slice_array<_Tp>::operator=(const slice_array& __sa) const
 {
     value_type* __t = __vp_;