BitField: Delete copy assignment to prevent obscure bugs.

Cf. https://github.com/dolphin-emu/dolphin/pull/483

src/common/bit_field.h

@@ -124,6 +124,22 @@ public:
     // so that we can use this within unions
     BitField() = default;
 
+#ifndef _WIN32
+    // We explicitly delete the copy assigment 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.
+    // MSVC 2013 has no support for this, hence we disable this code on
+    // Windows (so that the default copy assignment operator will be used).
+    // For any C++11 conformant compiler we delete the operator to make sure
+    // we never use this inappropriate operator to begin with.
+    // TODO: Implement this operator properly once all target compilers
+    // support unrestricted unions.
+    BitField& operator=(const BitField&) = delete;
+#endif
+
     __forceinline BitField& operator=(T val)
     {
         storage = (storage & ~GetMask()) | ((val << position) & GetMask());