Platform: add std._Private.xutility for Windows

This adds the `xutility` module to strip that from the first modular
inclusion of the header.

Author: compnerd

stdlib/public/Platform/vcruntime.modulemap

@@ -711,10 +711,6 @@ module std [system] {
       export *
     }
 
-    explicit module xtr1common {
-      header "xtr1common"
-      export *
-    }
 
     explicit module xstring {
       header "xstring"
@@ -725,5 +721,15 @@ module std [system] {
       header "xtree"
       export *
     }
+
+    explicit module xtr1common {
+      header "xtr1common"
+      export *
+    }
+
+    explicit module xutility {
+      header "xutility"
+      export *
+    }
   }
 }