[metadata] If a GTD appears uninstantiated in a generic instantiation argument,

[metadata] If a GTD appears uninstantiated in a generic instantiation argument,
replace it by its canonical open instantiation.

If we have
```
  class Foo<A,B,C> { ... }
  class Bar<T,U> { ... }
```

Then if someone tries to construct `Bar<Foo, int>` (or more precisely, the
MonoGenericInst corresponding to `<Foo, int>` instead return `Bar< Foo<A,B,C>, int >`.

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=54485