[libc++][NFC] Rename fold.h to ranges_fold.h (#109696)
This follows the pattern we use consistently for ranges algorithms.
This is a re-application of 24bc3244d4e which had been reverted in
f11abac65 due to unrelated failures.
diff --git a/libcxx/include/algorithm b/libcxx/include/algorithm
index 36fd035..17d63ce 100644
--- a/libcxx/include/algorithm
+++ b/libcxx/include/algorithm
@@ -2020,10 +2020,10 @@
#endif
#if _LIBCPP_STD_VER >= 23
-# include <__algorithm/fold.h>
# include <__algorithm/ranges_contains_subrange.h>
# include <__algorithm/ranges_ends_with.h>
# include <__algorithm/ranges_find_last.h>
+# include <__algorithm/ranges_fold.h>
# include <__algorithm/ranges_starts_with.h>
#endif // _LIBCPP_STD_VER >= 23
