[clangd][NFC] Make Located::operator->() use pointer sematics

This enables using the arrow operator to access members of the contained item.
```lang=c++
Located<std::string> X;
const char* CStr = X->c_str();
```
This is inline with how classes like `Optional` handle the arrow operator.

Reviewed By: kadircet

Differential Revision: https://reviews.llvm.org/D90682