[clang-tools-extra] [clangd] Add languages as server capabilities (PR #75633)

[Ashley Coleman via cfe-commits]

V-FEXrt wrote:

`ServerCapabilities` has this member:
```
         /**
	 * Experimental server capabilities.
	 */
	experimental?: LSPAny;
```

Any reason we wouldn't just use that directly? maybe  

```c++
  llvm::json::Object ServerCaps{
      ...
      {"experimental",
       llvm::json::Object{
           {"languages", {"HLSL"}},
       }},
      ...
  }
```

https://github.com/llvm/llvm-project/pull/75633