[stdlib] Dictionary.values: Replace setter with _modify

This should enable in-place modification of the Values view, which should greatly improve the performance of code like this:

dictionary.values[index] = newValue

Author: lorentey

stdlib/public/core/Dictionary.swift

@@ -1289,8 +1289,11 @@ extension Dictionary {
     get {
       return Values(_dictionary: self)
     }
-    set {
-      self._variant = newValue._variant
+    _modify {
+      var values = Values(_variant: _variant)
+      _variant = .native(_NativeDictionary())
+      yield &values
+      self._variant = values._variant
     }
   }