[libcxx] [test] Add another (void) cast for a function marked nodiscard in MS STL

[mstorsjo]

Summary

I had missed this one before, due to testing with overly broad ifdefs disabling parts of the tests...

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

for (auto& p : fail_cases)
             file_status st = symlink_status(p, ec);
             TEST_CHECK(ec == expect_ec);
             TEST_CHECK(st.type() == file_type::none);
             TEST_CHECK(st.permissions() == perms::unknown);
         }
 #ifndef TEST_HAS_NO_EXCEPTIONS
         { // test throwing case
             try {
-                symlink_status(p);
+                (void)symlink_status(p);
             } catch (filesystem_error const& err) {
                 TEST_CHECK(err.path1() == p);
                 TEST_CHECK(err.path2() == "");
                 TEST_CHECK(err.code() == expect_ec);
             }
         }
 #endif
     }