Automatically setting variant type when assigning values

`wil::unique_variant` could automatically set its `vt` member when assigning a value. I have 2 ways to approach this:
1. Overload `operator=` directly in `wil::unique_variant`, so assigning a value automatically sets `vt`

```cpp
wil::unique_variant value;
value = 42;
```
2. Make all union member a setter function, eg.

```cpp
wil::unique_variant value;
value.lVal(42);
```