[libc++] Implement P1272R4 (std::byteswap)

Implement P1274R4

Reviewed By: Quuxplusone, Mordante, #libc

Spies: jloser, lebedev.ri, mgorny, libcxx-commits, arichardson

Differential Revision: https://reviews.llvm.org/D114074

NOKEYCHECK=True
GitOrigin-RevId: 1dc62f2653f837745251bd905940c11962469b45
diff --git a/docs/FeatureTestMacroTable.rst b/docs/FeatureTestMacroTable.rst
index 8300b86..763e651 100644
--- a/docs/FeatureTestMacroTable.rst
+++ b/docs/FeatureTestMacroTable.rst
@@ -294,6 +294,8 @@
     ------------------------------------------------- -----------------
     **C++ 2b**
     -------------------------------------------------------------------
+    ``__cpp_lib_byteswap``                            ``202110L``
+    ------------------------------------------------- -----------------
     ``__cpp_lib_is_scoped_enum``                      ``202011L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_stacktrace``                          *unimplemented*
diff --git a/docs/Status/Cxx2bPapers.csv b/docs/Status/Cxx2bPapers.csv
index 1a1ca72..ae756c3 100644
--- a/docs/Status/Cxx2bPapers.csv
+++ b/docs/Status/Cxx2bPapers.csv
@@ -28,7 +28,7 @@
 "`P0849R8 <https://wg21.link/P0849R8>`__","LWG","``auto(x)``: ``DECAY_COPY`` in the language","October 2021","",""
 "`P1072R10 <https://wg21.link/P1072R10>`__","LWG","``basic_string::resize_and_overwrite``","October 2021","",""
 "`P1147R1 <https://wg21.link/P1147R1>`__","LWG","Printing ``volatile`` Pointers","October 2021","|Complete|","14.0"
-"`P1272R4 <https://wg21.link/P1272R4>`__","LWG","Byteswapping for fun&&nuf","October 2021","",""
+"`P1272R4 <https://wg21.link/P1272R4>`__","LWG","Byteswapping for fun&&nuf","October 2021","|Complete|","14.0"
 "`P1675R2 <https://wg21.link/P1675R2>`__","LWG","``rethrow_exception`` must be allowed to copy","October 2021","",""
 "`P2077R3 <https://wg21.link/P2077R3>`__","LWG","Heterogeneous erasure overloads for associative containers","October 2021","",""
 "`P2251R1 <https://wg21.link/P2251R1>`__","LWG","Require ``span`` & ``basic_string_view`` to be Trivially Copyable","October 2021","|Complete|","14.0"