Add return statement to slice_array and mask_array assignment. Closes PR20614.

This patch just adds the required return statements to slice_array::operator=
and mask_array::operator=.

Tests were added to check that the return value is the same as the object assigned
to.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@215414 91177308-0d34-0410-b5e6-96231b3b80d8

include/valarray

@@ -1220,6 +1220,7 @@ slice_array<_Tp>::operator=(const slice_array& __sa) const
     const value_type* __s = __sa.__vp_;
     for (size_t __n = __size_; __n; --__n, __t += __stride_, __s += __sa.__stride_)
         *__t = *__s;
+    return *this;
 }
 
 template <class _Tp>
@@ -2090,6 +2091,7 @@ mask_array<_Tp>::operator=(const mask_array& __ma) const
     size_t __n = __1d_.size();
     for (size_t __i = 0; __i < __n; ++__i)
         __vp_[__1d_[__i]] = __ma.__vp_[__1d_[__i]];
+    return *this;
 }
 
 template <class _Tp>