[clangd] Add desugared type to hover

[clangd] Add desugared type to hover

Add desugared type to hover when the desugared type and the pretty-printed type are different.

```c++
template<typename T>
struct TestHover {
  using Type = T;
};

int main() {
  TestHover<int>::Type a;
}
```

```
variable a

Type: TestHover<int>::Type (aka int)
```

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D114522