adds ranges::fold_left_with_iter and ranges::fold_left

Notable things in this commit:

* refactors `__indirect_binary_left_foldable`, making it slightly
different (but equivalent) to _`indirect-binary-left-foldable`_, which
improves readability (a [patch to the Working Paper][patch] was made)
* omits `__cpo` namespace, since it is not required for implementing
niebloids (a cleanup should happen in 2024)
* puts tests ensuring invocable robustness and dangling correctness
inside the correctness testing to ensure that the algorithms' results
are still correct

[patch]: cplusplus/draft#6734

Co-authored-by: Shelley Vohr <shelley.vohr@gmail.com>

Author: trop[bot]

filenames.libcxx.gni

@@ -24,6 +24,7 @@ libcxx_headers = [
   "//third_party/libc++/src/include/__algorithm/find_if.h",
   "//third_party/libc++/src/include/__algorithm/find_if_not.h",
   "//third_party/libc++/src/include/__algorithm/find_segment_if.h",
+  "//third_party/libc++/src/include/__algorithm/fold.h",
   "//third_party/libc++/src/include/__algorithm/for_each.h",
   "//third_party/libc++/src/include/__algorithm/for_each_n.h",
   "//third_party/libc++/src/include/__algorithm/for_each_segment.h",
@@ -109,6 +110,7 @@ libcxx_headers = [
   "//third_party/libc++/src/include/__algorithm/ranges_any_of.h",
   "//third_party/libc++/src/include/__algorithm/ranges_binary_search.h",
   "//third_party/libc++/src/include/__algorithm/ranges_clamp.h",
+  "//third_party/libc++/src/include/__algorithm/ranges_contains.h",
   "//third_party/libc++/src/include/__algorithm/ranges_copy.h",
   "//third_party/libc++/src/include/__algorithm/ranges_copy_backward.h",
   "//third_party/libc++/src/include/__algorithm/ranges_copy_if.h",
@@ -914,10 +916,8 @@ libcxx_headers = [
   "//third_party/libc++/src/include/expected",
   "//third_party/libc++/src/include/experimental/__config",
   "//third_party/libc++/src/include/experimental/__memory",
-  "//third_party/libc++/src/include/experimental/__simd/abi_tag.h",
   "//third_party/libc++/src/include/experimental/__simd/aligned_tag.h",
   "//third_party/libc++/src/include/experimental/__simd/declaration.h",
-  "//third_party/libc++/src/include/experimental/__simd/internal_declaration.h",
   "//third_party/libc++/src/include/experimental/__simd/reference.h",
   "//third_party/libc++/src/include/experimental/__simd/scalar.h",
   "//third_party/libc++/src/include/experimental/__simd/simd.h",