[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.cons/pointer_alloc.pass.cpp

@@ -8,7 +8,9 @@
 
 // <string>
 
-// basic_string(const charT* s, const Allocator& a = Allocator());
+// XFAIL: LIBCXX-AIX-FIXME
+
+// basic_string(const charT* s, const Allocator& a = Allocator()); // constexpr since C++20
 
 #include <string>
 #include <stdexcept>
@@ -51,7 +53,7 @@ test(const charT* s, const A& a)
     assert(s2.capacity() >= s2.size());
 }
 
-bool test() {
+TEST_CONSTEXPR_CXX20 bool test() {
     {
     typedef test_allocator<char> A;
 
@@ -92,7 +94,7 @@ int main(int, char**)
 {
   test();
 #if TEST_STD_VER > 17
-  // static_assert(test());
+  static_assert(test());
 #endif
 
   return 0;