[libc++] Implement P1423R3 (char8_t backward compatibility remediation)

Reviewed By: Mordante, #libc

Spies: h-vetinari, libcxx-commits, arichardson

Differential Revision: https://reviews.llvm.org/D129195

NOKEYCHECK=True
GitOrigin-RevId: 309aed306817e5b1a302d8f757231c734cf71370
diff --git a/docs/FeatureTestMacroTable.rst b/docs/FeatureTestMacroTable.rst
index f615b40..7ab2d6c 100644
--- a/docs/FeatureTestMacroTable.rst
+++ b/docs/FeatureTestMacroTable.rst
@@ -196,7 +196,7 @@
     ------------------------------------------------- -----------------
     ``__cpp_lib_bounded_array_traits``                ``201902L``
     ------------------------------------------------- -----------------
-    ``__cpp_lib_char8_t``                             ``201811L``
+    ``__cpp_lib_char8_t``                             ``201907L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_concepts``                            ``202002L``
     ------------------------------------------------- -----------------
diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst
index da76d39..138759a 100644
--- a/docs/ReleaseNotes.rst
+++ b/docs/ReleaseNotes.rst
@@ -49,6 +49,7 @@
 - P0618R0 (Deprecating <codecvt>)
 - P2418R2 (Add support for ``std::generator``-like types to ``std::format``)
 - LWG3659 (Consider ``ATOMIC_FLAG_INIT`` undeprecation)
+- P1423R3 (char8_t backward compatibility remediation)
 
 - Marked the following papers as "Complete" (note that some of those might have
   been implemented in a previous release but not marked as such):
diff --git a/docs/Status/Cxx20Papers.csv b/docs/Status/Cxx20Papers.csv
index 14129ad..703c4a5 100644
--- a/docs/Status/Cxx20Papers.csv
+++ b/docs/Status/Cxx20Papers.csv
@@ -115,7 +115,7 @@
 "`P1208R6 <https://wg21.link/P1208R6>`__","LWG","Adopt source_location for C++20","Cologne","",""
 "`P1355R2 <https://wg21.link/P1355R2>`__","LWG","Exposing a narrow contract for ceil2","Cologne","|Complete|","9.0"
 "`P1361R2 <https://wg21.link/P1361R2>`__","LWG","Integration of chrono with text formatting","Cologne","",""
-"`P1423R3 <https://wg21.link/P1423R3>`__","LWG","char8_t backward compatibility remediation","Cologne","|In Progress|",""
+"`P1423R3 <https://wg21.link/P1423R3>`__","LWG","char8_t backward compatibility remediation","Cologne","|Complete|","15.0"
 "`P1424R1 <https://wg21.link/P1424R1>`__","LWG","'constexpr' feature macro concerns","Cologne","Superseded by `P1902 <https://wg21.link/P1902>`__",""
 "`P1466R3 <https://wg21.link/P1466R3>`__","LWG","Miscellaneous minor fixes for chrono","Cologne","",""
 "`P1474R1 <https://wg21.link/P1474R1>`__","LWG","Helpful pointers for ContiguousIterator","Cologne","|Complete|","15.0"