[Index] Remove reference to `UnresolvedUsingIfExists`

Assuming `ns::foo` doesn't exist, given:
```
using ns::foo __attribute__((using_if_exists));
```

The AST will look something like:
UsingDecl
  UsingShadowDecl
    UnresolvedUsingIfExistsDecl

Thus we end up adding a reference to `UnresolvedUsingIfExistsDecl` when
processing `UsingDecl`, but never add the decl itself. In this case the
decl is really the `UsingDecl` anyway though (which we do output), so it
makes more sense to just remove the extra reference.

Differential Revision: https://reviews.llvm.org/D124288