[libc++] Add return 0 to some main() functions

This unbreaks the tests when running in freestanding mode.

Author: ldionne

libcxx/test/std/utilities/optional/optional.monadic/and_then.pass.cpp

@@ -259,4 +259,5 @@ constexpr bool test() {
 int main(int, char**) {
   test();
   static_assert(test());
+  return 0;
 }

libcxx/test/std/utilities/optional/optional.monadic/or_else.pass.cpp

@@ -69,4 +69,5 @@ constexpr bool test() {
 int main(int, char**) {
   test();
   static_assert(test());
+  return 0;
 }

libcxx/test/std/utilities/optional/optional.monadic/transform.pass.cpp

@@ -202,4 +202,5 @@ constexpr bool test() {
 int main(int, char**) {
   test();
   static_assert(test());
+  return 0;
 }