The "generic depth" is used to match up generic type variables.
For example:
```
struct Foo<T> { // `T` at generic depth 0
struct Bar {
struct Baz<U> { // 'U' at generic depth 1
...
}}}
```
Note in the above that `Bar` is not counted in the
generic depth. The previous logic did count `Bar` in
the generic depth calculation, leading to mismatches
when trying to associate references to generic variables.
This adds a new test with cases like the above and of course
corrects the calculation.
Resolves rdar://127450037
