Static variables used in a default argument context are now emitted using
their fully qualified name. This is to avoid generating ambiguous
assignments, for instance:
```
struct S
{
__gshared int param;
void example(int param = S.param);
}
```
In the above example, the visitor for dtoh VarExp will omit the `S`
parent, as the static field is nested in `S`.
```
void example(int param = param);
```
Although D understands that `param` refers to the static field, this is
not the case in C++, and the compilation will fail with:
```
error: parameter 'param' may not appear in this context
```
