[libc++][NFC] Fix incorrect main() signatures

Author: ldionne

libcxx/test/std/containers/sequences/vector/vector.modifiers/destroy_elements.pass.cpp

@@ -64,9 +64,11 @@ TEST_CONSTEXPR_CXX20 bool test() {
   return true;
 }
 
-int main() {
+int main(int, char**) {
   test();
 #if TEST_STD_VER >= 20
   static_assert(test());
 #endif
+
+  return 0;
 }

libcxx/test/std/numerics/c.math/hermite.pass.cpp

@@ -350,7 +350,9 @@ struct TestInt {
   }
 };
 
-int main() {
+int main(int, char**) {
   types::for_each(types::floating_point_types(), TestFloat());
   types::for_each(types::type_list<short, int, long, long long>(), TestInt());
+
+  return 0;
 }

libcxx/test/std/time/time.zone/time.zone.info/time.zone.info.local/local_info.members.pass.cpp

@@ -31,7 +31,7 @@
 #include <string>
 #include <type_traits>
 
-int main(int, const char**) {
+int main(int, char**) {
   {
     constexpr const int& result = std::chrono::local_info::unique;
     static_assert(result == 0);

libcxx/test/std/time/time.zone/time.zone.info/time.zone.info.sys/sys_info.members.pass.cpp

@@ -28,7 +28,7 @@
 #include <string>
 #include <type_traits>
 
-int main(int, const char**) {
+int main(int, char**) {
   static_assert(std::is_aggregate_v<std::chrono::sys_info>);
 
   std::chrono::sys_info sys_info{