[libcxx] [test] Add (void) return casts on functions marked nodiscard in MS STL

libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.canonical/canonical.pass.cpp

 
 TEST_CASE(test_exception_contains_paths)
 {
 #ifndef TEST_HAS_NO_EXCEPTIONS
     static_test_env static_env;
     CWDGuard guard;
     const path p = "blabla/dne";
     try {
-        canonical(p);
+        (void)canonical(p);
         TEST_REQUIRE(false);
     } catch (filesystem_error const& err) {
         TEST_CHECK(err.path1() == p);
         // libc++ provides the current path as the second path in the exception
         LIBCPP_ONLY(TEST_CHECK(err.path2() == current_path()));
     }
     fs::current_path(static_env.Dir);
     try {
-        canonical(p);
+        (void)canonical(p);
         TEST_REQUIRE(false);
     } catch (filesystem_error const& err) {
         TEST_CHECK(err.path1() == p);
         LIBCPP_ONLY(TEST_CHECK(err.path2() == static_env.Dir));
     }
 #endif
 }
 
 TEST_SUITE_END()

libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.copy_file/copy_file.pass.cpp

TEST_CASE(non_regular_file_test) {
   }
 
 }
 
 TEST_CASE(test_attributes_get_copied) {
   scoped_test_env env;
   const path file = env.create_file("file1", 42);
   const path dest = env.make_env_path("file2");
-  auto st = status(file);
+  (void)status(file);
   perms new_perms = perms::owner_read;
   permissions(file, new_perms);
   std::error_code ec = GetTestEC();
   TEST_REQUIRE(fs::copy_file(file, dest, ec) == true);
   TEST_CHECK(!ec);
   auto new_st = status(dest);
   TEST_CHECK(new_st.permissions() == new_perms);
 }

libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.read_symlink/read_symlink.pass.cpp

 }
 
 TEST_CASE(test_error_reporting)
 {
     auto checkThrow = [](path const& f, const std::error_code& ec)
     {
 #ifndef TEST_HAS_NO_EXCEPTIONS
         try {
-            fs::read_symlink(f);
+            (void)fs::read_symlink(f);
             return false;
         } catch (filesystem_error const& err) {
             return err.path1() == f
                 && err.path2() == ""
                 && err.code() == ec;
         }
 #else
         ((void)f); ((void)ec);

libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.space/space.pass.cpp

 
 TEST_CASE(test_error_reporting)
 {
     static_test_env static_env;
     auto checkThrow = [](path const& f, const std::error_code& ec)
     {
 #ifndef TEST_HAS_NO_EXCEPTIONS
         try {
-            space(f);
+            (void)space(f);
             return false;
         } catch (filesystem_error const& err) {
             return err.path1() == f
                 && err.path2() == ""
                 && err.code() == ec;
         }
 #else
         ((void)f); ((void)ec);