[libc++] Remove the chrono include from algorithm

algorithm's include of chrono causes include cycles:

```
algorithm -> chrono -> __chrono/convert_to_tm.h -> __chrono/statically_widen.h -> __format/concepts.h -> __format/format_parse_context.h -> string_view -> algorithm

algorithm -> chrono -> __chrono/convert_to_tm.h -> __chrono/statically_widen.h -> __format/concepts.h -> __format/format_parse_context.h -> string_view -> functional -> __functional/boyer_moore_searcher.h -> array -> algorithm

algorithm -> chrono -> __chrono/convert_to_tm.h -> __chrono/statically_widen.h -> __format/concepts.h -> __format/format_parse_context.h -> string_view -> functional -> __functional/boyer_moore_searcher.h -> unordered_map -> algorithm

algorithm -> chrono -> __chrono/convert_to_tm.h -> __chrono/statically_widen.h -> __format/concepts.h -> __format/format_parse_context.h -> string_view -> functional -> __functional/boyer_moore_searcher.h -> vector -> algorithm
```

This is a problem for clang modules after the std mega module is broken up, because it becomes a module cycle which is a hard error.

All of the includes in the `__chrono` and `__format` headers are being used and so can't be removed. algorithm's include of chrono is already removed in C++20, whereas the array, string_view, unordered_map, vector includes of algorithm aren't removed until C++23 (and it's 4x the includes that would need removing). Unconditionally remove the chrono include from algorithm in all versions, so that the module breakup can happen (the module has to apply to all C++ versions).

Reviewed By: Mordante, #libc

Differential Revision: https://reviews.llvm.org/D148405

libcxx/docs/ReleaseNotes.rst

@@ -67,6 +67,8 @@ Deprecations and Removals
   - C++2b: ``atomic``, ``bit``, ``cstdint``, ``cstdlib``, ``cstring``, ``initializer_list``, ``limits``, ``new``,
            ``stdexcept``, ``system_error``, ``type_traits``, ``typeinfo``
 
+- ``<algorithm>`` no longer includes ``<chrono>`` in any C++ version (it was prevously included in C++17 and earlier).
+
 - The headers ``<experimental/algorithm>`` and ``<experimental/functional>`` have been removed, since all the contents
   have been implemented in namespace ``std`` for at least two releases.
 

libcxx/include/algorithm

@@ -1917,10 +1917,6 @@ template <class BidirectionalIterator, class Compare>
 #  pragma GCC system_header
 #endif
 
-#if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 17
-#  include <chrono>
-#endif
-
 #if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
 #  include <atomic>
 #  include <bit>

libcxx/test/libcxx/transitive_includes/cxx03.csv

@@ -1,6 +1,5 @@
 algorithm atomic
 algorithm bit
-algorithm chrono
 algorithm climits
 algorithm concepts
 algorithm cstddef
@@ -854,6 +853,7 @@ thread chrono
 thread compare
 thread cstddef
 thread cstdint
+thread cstdlib
 thread cstring
 thread ctime
 thread functional

libcxx/test/libcxx/transitive_includes/cxx11.csv

@@ -1,6 +1,5 @@
 algorithm atomic
 algorithm bit
-algorithm chrono
 algorithm climits
 algorithm concepts
 algorithm cstddef
@@ -855,6 +854,7 @@ thread chrono
 thread compare
 thread cstddef
 thread cstdint
+thread cstdlib
 thread cstring
 thread ctime
 thread functional

libcxx/test/libcxx/transitive_includes/cxx14.csv

@@ -1,6 +1,5 @@
 algorithm atomic
 algorithm bit
-algorithm chrono
 algorithm climits
 algorithm concepts
 algorithm cstddef
@@ -857,6 +856,7 @@ thread chrono
 thread compare
 thread cstddef
 thread cstdint
+thread cstdlib
 thread cstring
 thread ctime
 thread functional

libcxx/test/libcxx/transitive_includes/cxx17.csv

@@ -1,6 +1,5 @@
 algorithm atomic
 algorithm bit
-algorithm chrono
 algorithm climits
 algorithm concepts
 algorithm cstddef
@@ -857,6 +856,7 @@ thread chrono
 thread compare
 thread cstddef
 thread cstdint
+thread cstdlib
 thread cstring
 thread ctime
 thread functional