[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

libcxx/test/std/strings/basic.string/string.modifiers/string_append/string.pass.cpp

@@ -6,10 +6,12 @@
 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: LIBCXX-AIX-FIXME
+
 // <string>
 
 // basic_string<charT,traits,Allocator>&
-//   append(const basic_string<charT,traits>& str);
+//   append(const basic_string<charT,traits>& str); // constexpr since C++20
 
 #include <string>
 #include <cassert>
@@ -26,7 +28,7 @@ test(S s, S str, S expected)
     assert(s == expected);
 }
 
-bool test() {
+TEST_CONSTEXPR_CXX20 bool test() {
   {
     typedef std::string S;
     test(S(), S(), S());
@@ -92,7 +94,7 @@ int main(int, char**)
 {
   test();
 #if TEST_STD_VER > 17
-  // static_assert(test());
+  static_assert(test());
 #endif
 
   return 0;