[clangd] Report xref for base methods.

See: https://github.com/clangd/clangd/issues/668

```
struct A { virtual void foo() = 0; };
struct B : A { void foo() override; };
```

Find refs on `B::foo()` will show:
- decls of `A::foo()` (new)
- decls of `B::foo()`
- refs to `A::foo()` (new)
- refs to `B::foo()`.

Differential Revision: https://reviews.llvm.org/D95852