[libcxx] [test] Fix shadow warnings.

[STL_MSFT]

Summary

Fix shadow warnings. This variable was scoped incorrectly, found by MSVC.

[EricWF]

Y'all need to get commit access :-P

[EricWF]

r284941.

test/std/utilities/optional/optional.object/optional.object.assign/emplace.pass.cpp

 }
 
 
 template <class T>
 void test_multi_arg()
 {
     test_one_arg<T>();
     using Opt = std::optional<T>;
-    Opt opt;
     {
+        Opt opt;
         opt.emplace(101, 41);
         assert(static_cast<bool>(opt) == true);
         assert(*opt == T(101, 41));
     }
     {
         Opt opt;
         opt.emplace({1, 2, 3, 4});
         assert(static_cast<bool>(opt) == true);