Strengthen storage variables being unassigned now that we have inline assembly

Reported by @magnus237:

The following code does not report a warning because of the self-assignment:
```
function f() external pure {
string storage s1;
s1 = s1; // self-assignment
string storage s2 = s1;
}
```

Now that we have inline-assembly access to the slots, the following code would be a better (more explicit) way of saying "please ignore the warning":
```
function f() external pure {
string storage s1;
assembly { s1.slot := 0 }
string storage s2 = s1;
}
```

Can we change the control-flow-analyzer accordingly, i.e. do not take assignments from variables into account which are themselves not yet initialized?

Start by locating the control-flow analyzer and reproduce the two Solidity examples from the issue, comparing the warning produced for the self-assignment with the inline-assembly form. Done means assignments from storage variables that are not initialized no longer suppress the unassigned-variable warning, while the explicit slot assignment provides the intended way to suppress it.

Written by the indexing model from the issue text.