[clang] [clang][C2x] Remove confusing diagnostic auto storage class specifier (PR #68710)

[Guillot Tony via cfe-commits]

to268 wrote:

Nope, I can't
> This branch has no conflicts with the base branch
Only those with [write access](https://docs.github.com/articles/what-are-the-different-access-permissions) to this repository can merge pull requests.

https://github.com/llvm/llvm-project/pull/68710