[clang-tools-extra] [clangd] Include the operator name in documentHighlight (PR #220518)

[Christian Kandeler via cfe-commits]

ckandeler wrote:

> Sorry for didn't mention this before. I found one more case when behavior is not expected
> 
> ```c++
> struct S {
>   void [[operator +]](int);          // removing this line leads to expected behavior
>   void [[operat^or +]](double);
> };
> 
> template <typename T>
> void foo(S s, T t) {
>   s.[[operator]] +(t); // operator is highlighted, but + is not
> }
> ```

Fixed.

https://github.com/llvm/llvm-project/pull/220518