[libc++] Add missing `return 0` to main() functions in the tests

libcxx/test/libcxx/ranges/range.access/begin.incomplete_type.sh.cpp

@@ -69,6 +69,7 @@ int main(int, char**)
 {
     test();
     static_assert(test());
+    return 0;
 }
 
 #endif // defined(TU1)

libcxx/test/libcxx/ranges/range.access/end.incomplete_type.pass.cpp

@@ -43,4 +43,5 @@ int main(int, char**)
 {
     test();
     static_assert(test());
+    return 0;
 }