_Atomic auto and _Atomic(auto) are not the same thing in C23

_This comes from discussion in https://github.com/llvm/llvm-project/pull/197874#pullrequestreview-4389392176_

`_Atomic auto` is using `_Atomic` as a qualifier, while `_Atomic(auto)` is a type specifier spelling. The qualifier form should be accepted in C23 mode but Clang rejects the same as the type specifier form: https://godbolt.org/z/1453Yz4r5

Reproduce the distinction between `_Atomic auto` and `_Atomic(auto)` in C23 mode using the Godbolt example linked in the issue. Read the discussion in LLVM pull request 197874 to understand the intended parsing and semantics. Done means the qualifier spelling is accepted while the type-specifier spelling retains its distinct behavior, with regression coverage for both forms.

Written by the indexing model from the issue text.