Deprecate no-op assignments

Who hasn't been bitten by this bug?
```D
struct Foo {
  int bar;
  this(int bar) {
    bar = bar;
  }
}
```
Instead of assigning the constructor parameter to the struct member as intended, this does nothing. Oops.

As long as there's no opAssign overload involved, these mistakes are guaranteed to be no-ops. Let's make them errors instead.