Don't delete BitField copy assignment operator on VS

Source/Core/Common/BitField.h

@@ -124,15 +124,18 @@ public:
   // so that we can use this within unions
   constexpr BitField() = default;
 
+// Visual Studio (as of VS2017) considers BitField to not be trivially
+// copyable if we delete this copy assignment operator.
+// https://developercommunity.visualstudio.com/content/problem/101208/c-compiler-is-overly-strict-regarding-whether-a-cl.html
+#ifndef _MSC_VER
   // We explicitly delete the copy assignment operator here, because the
   // default copy assignment would copy the full storage value, rather than
   // just the bits relevant to this particular bit field.
   // Ideally, we would just implement the copy assignment to copy only the
-  // relevant bits, but this requires compiler support for unrestricted
-  // unions.
-  // TODO: Implement this operator properly once all target compilers
-  // support unrestricted unions.
+  // relevant bits, but we're prevented from doing that because the savestate
+  // code expects that this class is trivially copyable.
   BitField& operator=(const BitField&) = delete;
+#endif
 
   __forceinline BitField& operator=(T val)
   {