[clang] [clang][C23] N3006 Underspecified object declarations (PR #79845)

[Guillot Tony via cfe-commits]

to268 wrote:

I have rebased my PR to include N3018 constexpr feature and reverted a formatted snippet of code that is not part of the PR.
I will add constexpr tests but I need to investigate why the constexpr keyword does not trigger the diagnostic (not sure if it's a constexpr implementation bug or something I need to handle in my PR (it does not really matter TBO)).

https://github.com/llvm/llvm-project/pull/79845