[LLD][MachO] Silence warning when building with MSVC

Silence the following warning with latest MSVC:
```
C:\git\llvm-project\lld\MachO\Driver.cpp(358): warning C4189: 't': local variable is initialized but not referenced
```

Author: aganea

lld/MachO/Driver.cpp

@@ -354,8 +354,10 @@ void multiThreadedPageInBackground(DeferredFiles &deferred) {
 
     // Reference all file's mmap'd pages to load them into memory.
     for (const char *page = buff.data(), *end = page + buff.size(); page < end;
-         page += pageSize)
+         page += pageSize) {
       LLVM_ATTRIBUTE_UNUSED volatile char t = *page;
+      (void)t;
+    }
   };
 #if LLVM_ENABLE_THREADS
   { // Create scope for waiting for the taskGroup