Fix a -Wunused-variable warning on windows (llvm#157973)

The `UID` variable is only being used inside the `#ifndef _WIN32` block - move
the definition into the block as well.

Author: bogner

llvm/unittests/Support/VirtualOutputBackendsTest.cpp

@@ -559,9 +559,9 @@ Error OnDiskOutputBackendProvider::checkKept(StringRef FilePath,
   if (Error E = lookupFileInfo(FilePath, Info))
     return E;
 
+#ifndef _WIN32
   sys::fs::UniqueID UID =
       shouldUseTemporaries(*Info) ? *Info->TempUID : *Info->UID;
-#ifndef _WIN32
   if (!Info->F->hasUniqueID(UID))
     return createStringError(inconvertibleErrorCode(),
                              "File not created by keep or changed UID");