[libc++] Implement P0980R1 (constexpr std::string)

Reviewed By: #libc, ldionne

Spies: daltenty, sdasgup3, ldionne, arichardson, MTC, ChuanqiXu, mehdi_amini, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, aartbik, liufengdb, stephenneuendorffer, Joonsoo, grosul1, Kayjukh, jurahul, msifontes, tatianashp, rdzhabarov, teijeong, cota, dcaballe, Chia-hungDuan, wrengr, wenzhicui, arphaman, Mordante, miscco, Quuxplusone, smeenai, libcxx-commits

Differential Revision: https://reviews.llvm.org/D110598

NOKEYCHECK=True
GitOrigin-RevId: 425620ccdd47e56b59512913cdc71e116f951e4e
diff --git a/docs/FeatureTestMacroTable.rst b/docs/FeatureTestMacroTable.rst
index 792bbd0..4f2ce96 100644
--- a/docs/FeatureTestMacroTable.rst
+++ b/docs/FeatureTestMacroTable.rst
@@ -214,7 +214,7 @@
     ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_numeric``                   ``201911L``
     ------------------------------------------------- -----------------
-    ``__cpp_lib_constexpr_string``                    ``201811L``
+    ``__cpp_lib_constexpr_string``                    ``201907L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_string_view``               ``201811L``
     ------------------------------------------------- -----------------
diff --git a/docs/Status/Cxx20Papers.csv b/docs/Status/Cxx20Papers.csv
index fa88a5a..65e4014 100644
--- a/docs/Status/Cxx20Papers.csv
+++ b/docs/Status/Cxx20Papers.csv
@@ -106,7 +106,7 @@
 "`P0645R10 <https://wg21.link/P0645R10>`__","LWG","Text Formatting","Cologne","|Complete| [#note-P0645]_","14.0"
 "`P0660R10 <https://wg21.link/P0660R10>`__","LWG","Stop Token and Joining Thread, Rev 10","Cologne","",""
 "`P0784R7 <https://wg21.link/P0784R7>`__","CWG","More constexpr containers","Cologne","|Complete|","12.0"
-"`P0980R1 <https://wg21.link/P0980R1>`__","LWG","Making std::string constexpr","Cologne","",""
+"`P0980R1 <https://wg21.link/P0980R1>`__","LWG","Making std::string constexpr","Cologne","|Complete|","15.0"
 "`P1004R2 <https://wg21.link/P1004R2>`__","LWG","Making std::vector constexpr","Cologne","",""
 "`P1035R7 <https://wg21.link/P1035R7>`__","LWG","Input Range Adaptors","Cologne","",""
 "`P1065R2 <https://wg21.link/P1065R2>`__","LWG","Constexpr INVOKE","Cologne","|Complete|","12.0"