[libc++][NFC] Use __constexpr_memmove instead of copy_n in <__string/char_traits.h> (#85920)

`copy_n` has been used to allow constant evaluation of `char_traits`. We
now have `__constexpr_memmove`, which `copy_n` just forwards to. We can
call `__constexpr_memmove` directly, avoiding a bunch of instantiations.
This reduces the time it takes to include `<string>` from 321ms to
285ms.

Author: philnik777

libcxx/include/__string/char_traits.h

@@ -9,7 +9,6 @@
 #ifndef _LIBCPP___STRING_CHAR_TRAITS_H
 #define _LIBCPP___STRING_CHAR_TRAITS_H
 
-#include <__algorithm/copy_n.h>
 #include <__algorithm/fill_n.h>
 #include <__algorithm/find_end.h>
 #include <__algorithm/find_first_of.h>
@@ -144,7 +143,7 @@ struct _LIBCPP_TEMPLATE_VIS char_traits<char> {
   copy(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT {
     _LIBCPP_ASSERT_NON_OVERLAPPING_RANGES(!std::__is_pointer_in_range(__s1, __s1 + __n, __s2),
                                           "char_traits::copy: source and destination ranges overlap");
-    std::copy_n(__s2, __n, __s1);
+    std::__constexpr_memmove(__s1, __s2, __element_count(__n));
     return __s1;
   }
 
@@ -221,7 +220,7 @@ struct _LIBCPP_TEMPLATE_VIS char_traits<wchar_t> {
   copy(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT {
     _LIBCPP_ASSERT_NON_OVERLAPPING_RANGES(!std::__is_pointer_in_range(__s1, __s1 + __n, __s2),
                                           "char_traits::copy: source and destination ranges overlap");
-    std::copy_n(__s2, __n, __s1);
+    std::__constexpr_memmove(__s1, __s2, __element_count(__n));
     return __s1;
   }
 
@@ -287,7 +286,7 @@ struct _LIBCPP_TEMPLATE_VIS char_traits<char8_t> {
   copy(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT {
     _LIBCPP_ASSERT_NON_OVERLAPPING_RANGES(!std::__is_pointer_in_range(__s1, __s1 + __n, __s2),
                                           "char_traits::copy: source and destination ranges overlap");
-    std::copy_n(__s2, __n, __s1);
+    std::__constexpr_memmove(__s1, __s2, __element_count(__n));
     return __s1;
   }
 
@@ -366,7 +365,7 @@ struct _LIBCPP_TEMPLATE_VIS char_traits<char16_t> {
   copy(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT {
     _LIBCPP_ASSERT_NON_OVERLAPPING_RANGES(!std::__is_pointer_in_range(__s1, __s1 + __n, __s2),
                                           "char_traits::copy: source and destination ranges overlap");
-    std::copy_n(__s2, __n, __s1);
+    std::__constexpr_memmove(__s1, __s2, __element_count(__n));
     return __s1;
   }
 
@@ -454,7 +453,7 @@ struct _LIBCPP_TEMPLATE_VIS char_traits<char32_t> {
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 static char_type*
   copy(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT {
-    std::copy_n(__s2, __n, __s1);
+    std::__constexpr_memmove(__s1, __s2, __element_count(__n));
     return __s1;
   }