[range.lazy.split.inner] Fix "base()" according to LWG3533

This is a follow-up to LWG3533 ("Consistency for `base() const &`"),
which changes lazy_split_view::inner-iterator::base's return type
and requires-clause.

Author: tkoeppe

source/ranges.tex

@@ -5993,8 +5993,7 @@
     @\exposid{inner-iterator}@() = default;
     constexpr explicit @\exposid{inner-iterator}@(@\exposid{outer-iterator}@<Const> i);
 
-    constexpr iterator_t<@\exposid{Base}@> base() const&
-      requires @\libconcept{copyable}@<iterator_t<@\exposid{Base}@>>;
+    constexpr const iterator_t<@\exposid{Base}@>& base() const &;
     constexpr iterator_t<@\exposid{Base}@> base() &&;
 
     constexpr decltype(auto) operator*() const { return *@\exposid{i_}@.@\placeholder{current}@; }
@@ -6051,8 +6050,7 @@
 
 \indexlibrarymember{base}{lazy_split_view::\exposid{inner-iterator}}%
 \begin{itemdecl}
-constexpr iterator_t<@\exposid{Base}@> base() const&
-  requires @\libconcept{copyable}@<iterator_t<@\exposid{Base}@>>;
+constexpr const iterator_t<@\exposid{Base}@>& base() const &;
 \end{itemdecl}
 
 \begin{itemdescr}