[libc++] Implement P0429R9 std::flat_map (#98643)

Around half of the tests are based on the tests Arthur O'Dwyer's
original implementation of std::flat_map, with modifications and
removals.

partially implement #105190

Author: huixie90

libcxx/docs/ReleaseNotes/20.rst

@@ -43,6 +43,7 @@ Implemented Papers
 - P2985R0: A type trait for detecting virtual base classes (`Github <https://github.com/llvm/llvm-project/issues/105432>`__)
 - ``std::jthread`` and ``<stop_token>`` are not guarded behind ``-fexperimental-library`` anymore
 - P2674R1: A trait for implicit lifetime types (`Github <https://github.com/llvm/llvm-project/issues/105259>`__)
+- P0429R9: A Standard ``flat_map`` is partially implemented and ``flat_map`` is provided (`Github <https://github.com/llvm/llvm-project/issues/105190>`__)
 
 Improvements and New Features
 -----------------------------

libcxx/docs/Status/Cxx23Papers.csv

@@ -52,7 +52,7 @@
 "`P2443R1 <https://wg21.link/P2443R1>`__","``views::chunk_by``","2022-02 (Virtual)","|Complete|","18.0",""
 "","","","","",""
 "`P0009R18 <https://wg21.link/P0009R18>`__","mdspan: A Non-Owning Multidimensional Array Reference","2022-07 (Virtual)","|Complete|","18.0",""
-"`P0429R9 <https://wg21.link/P0429R9>`__","A Standard ``flat_map``","2022-07 (Virtual)","","",""
+"`P0429R9 <https://wg21.link/P0429R9>`__","A Standard ``flat_map``","2022-07 (Virtual)","|In progress|","",""
 "`P1169R4 <https://wg21.link/P1169R4>`__","``static operator()``","2022-07 (Virtual)","|Complete|","16.0",""
 "`P1222R4 <https://wg21.link/P1222R4>`__","A Standard ``flat_set``","2022-07 (Virtual)","","",""
 "`P1223R5 <https://wg21.link/P1223R5>`__","``ranges::find_last()``, ``ranges::find_last_if()``, and ``ranges::find_last_if_not()``","2022-07 (Virtual)","|Complete|","19.0",""

libcxx/include/CMakeLists.txt

@@ -358,6 +358,8 @@ set(files
   __filesystem/recursive_directory_iterator.h
   __filesystem/space_info.h
   __filesystem/u8path.h
+  __flat_map/flat_map.h
+  __flat_map/sorted_unique.h
   __format/buffer.h
   __format/concepts.h
   __format/container_adaptor.h
@@ -959,6 +961,7 @@ set(files
   ext/hash_set
   fenv.h
   filesystem
+  flat_map
   float.h
   format
   forward_list