DLS will fail to bind methods to default declarations when they contain invalid defaults

Replicates via

```
template foo {
method foo_method() default {
default(); // invalid, but should not break overrides from below code
}
}

template foo_2 is foo {
method foo_method() {
default(); // goto-def here should find foo_method in foo template, but does not
}
}

is foo_2;
```

Removing the 'default' decl in foo_method fixes the issue. From logs the difference is that we fail to map the cursor position to a symbol in the failing case.