[libc++] Fix a typo in reverse_iterator::operator=.

We should be checking `is_assignable<It&, ...>`.
`is_assignable<It, ...>` checks for an rvalue left-hand side, which
is basically never assignable-to.
Found while looking into https://cplusplus.github.io/LWG/issue3435 .

Differential Revision: https://reviews.llvm.org/D117660

libcxx/include/__iterator/reverse_iterator.h

@@ -77,7 +77,7 @@ _LIBCPP_SUPPRESS_DEPRECATED_POP
     template <class _Up, class = __enable_if_t<
         !is_same<_Up, _Iter>::value &&
         is_convertible<_Up const&, _Iter>::value &&
-        is_assignable<_Iter, _Up const&>::value
+        is_assignable<_Iter&, _Up const&>::value
     > >
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
     reverse_iterator& operator=(const reverse_iterator<_Up>& __u) {
@@ -102,7 +102,7 @@ _LIBCPP_SUPPRESS_DEPRECATED_POP
     template <class _Up, class = __enable_if_t<
         !is_same<_Up, _Iter>::value &&
         is_convertible<_Up const&, _Iter>::value &&
-        is_assignable<_Iter, _Up const&>::value
+        is_assignable<_Iter&, _Up const&>::value
     > >
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
     reverse_iterator& operator=(const reverse_iterator<_Up>& __u) {