[libc++] [ranges] Add namespace __cpo to ranges::{advance,next,prev}.

The reason for those nested namespaces is explained in D115315:

> AIUI, this keeps the CPO's own type from ADL'ing into the `std::ranges`
> namespace; e.g. `foobar(std::ranges::uninitialized_default_construct)`
> should not consider `std::ranges::foobar` a candidate, even if
> `std::ranges::foobar` is not a CPO itself. Also, of course, consistency
> (Chesterton's Fence, the economist's hundred-dollar bill): if it were
> safe to omit the namespace, we'd certainly want to do it everywhere,
> not just here.

This makes these three niebloids more consistent with the other Ranges
niebloids we've already implemented, such as the `ranges::begin` group
and the `ranges::uninitialized_default_construct` group.

FWIW, we still have three different indentation-and-comment styles
among these three groups.

Differential Revision: https://reviews.llvm.org/D116569

libcxx/include/__iterator/advance.h

@@ -67,10 +67,12 @@ void advance(_InputIter& __i, _Distance __orig_n) {
 
 #if !defined(_LIBCPP_HAS_NO_RANGES)
 
-namespace ranges {
 // [range.iter.op.advance]
-// TODO(varconst): rename `__advance_fn` to `__fn`.
-struct __advance_fn final : private __function_like {
+
+namespace ranges {
+namespace __advance {
+
+struct __fn final : private __function_like {
 private:
   template <class _Tp>
   _LIBCPP_HIDE_FROM_ABI
@@ -97,7 +99,7 @@ private:
   }
 
 public:
-  constexpr explicit __advance_fn(__tag __x) noexcept : __function_like(__x) {}
+  constexpr explicit __fn(__tag __x) noexcept : __function_like(__x) {}
 
   // Preconditions: If `I` does not model `bidirectional_iterator`, `n` is not negative.
   template <input_or_output_iterator _Ip>
@@ -186,7 +188,11 @@ public:
   }
 };
 
-inline constexpr auto advance = __advance_fn(__function_like::__tag());
+} // namespace __advance
+
+inline namespace __cpo {
+  inline constexpr auto advance = __advance::__fn(__function_like::__tag());
+} // namespace __cpo
 } // namespace ranges
 
 #endif // !defined(_LIBCPP_HAS_NO_RANGES)

libcxx/include/__iterator/next.h

@@ -38,11 +38,14 @@ inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
 
 #if !defined(_LIBCPP_HAS_NO_RANGES)
 
+// [range.iter.op.next]
+
 namespace ranges {
-// TODO(varconst): rename `__next_fn` to `__fn`.
-struct __next_fn final : private __function_like {
+namespace __next {
+
+struct __fn final : private __function_like {
   _LIBCPP_HIDE_FROM_ABI
-  constexpr explicit __next_fn(__tag __x) noexcept : __function_like(__x) {}
+  constexpr explicit __fn(__tag __x) noexcept : __function_like(__x) {}
 
   template <input_or_output_iterator _Ip>
   _LIBCPP_HIDE_FROM_ABI
@@ -73,7 +76,11 @@ struct __next_fn final : private __function_like {
   }
 };
 
-inline constexpr auto next = __next_fn(__function_like::__tag());
+} // namespace __next
+
+inline namespace __cpo {
+  inline constexpr auto next = __next::__fn(__function_like::__tag());
+} // namespace __cpo
 } // namespace ranges
 
 #endif // !defined(_LIBCPP_HAS_NO_RANGES)

libcxx/include/__iterator/prev.h

@@ -37,11 +37,14 @@ inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
 
 #if !defined(_LIBCPP_HAS_NO_RANGES)
 
+// [range.iter.op.prev]
+
 namespace ranges {
-// TODO(varconst): rename `__prev_fn` to `__fn`.
-struct __prev_fn final : private __function_like {
+namespace __prev {
+
+struct __fn final : private __function_like {
   _LIBCPP_HIDE_FROM_ABI
-  constexpr explicit __prev_fn(__tag __x) noexcept : __function_like(__x) {}
+  constexpr explicit __fn(__tag __x) noexcept : __function_like(__x) {}
 
   template <bidirectional_iterator _Ip>
   _LIBCPP_HIDE_FROM_ABI
@@ -65,7 +68,11 @@ struct __prev_fn final : private __function_like {
   }
 };
 
-inline constexpr auto prev = __prev_fn(__function_like::__tag());
+} // namespace __prev
+
+inline namespace __cpo {
+  inline constexpr auto prev = __prev::__fn(__function_like::__tag());
+} // namespace __cpo
 } // namespace ranges
 
 #endif // !defined(_LIBCPP_HAS_NO_RANGES)