[clang-tools-extra] Refactor clang doc comment structure (PR #142273)

[Samarth Narang via cfe-commits]

snarang181 wrote:

> LGTM once the last few nits are addressed.

@ilovepi, for the last few nits,

- Removed the `default` case in the switch, and manually returning `nullptr` for all the unhandled cases. 
- Removed the braces in the `case` of the `switch` and shifted the variable decl. inside outside the switch to get around the compiler errors. 

https://github.com/llvm/llvm-project/pull/142273