[clang][index] Index unresolved member expression as reference

Unresolved member expressions aren't indexed as references.

Example code:

```
struct Foo {
  template <typename T> void bar(T t);
};
template <typename T> void test(Foo F, T t) {
  F.bar(t); // Not indexed
}
```

Reviewed By: hokein

Differential Revision: https://reviews.llvm.org/D131091