[clang-tools-extra] [clangd] Add inlay hints for default function arguments (PR #95712)

[Tor Shepherd via cfe-commits]

torshepherd wrote:

> Hmm another case to consider:
> 
> ![image](https://private-user-images.githubusercontent.com/49597791/372187742-193c9570-0e6b-47cd-8d8c-0279e727eac2.png?jwt=eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.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.1oEmnUxc-hEikrOjLY9iZvGMqaQp8acwcrfB1bDayT0)
> 
> In this case the argument is unnamed. I think for this I should just insert `/* unused */` or `/* unnamed */` ?

Fixed this and added a test case

![image](https://github.com/user-attachments/assets/49e29147-a2ed-4482-b2d5-d0b2996fdfd2)


https://github.com/llvm/llvm-project/pull/95712