[clang] [clang] Use internal linkage for c23 constexpr vars. (PR #97846)

[Mariya Podchishchaeva via cfe-commits]

Fznamznon wrote:

> LGTM but please add a release note to clang/docs/ReleaseNotes.rst so users know about the fix as well.

Note that only clang 19 has `constexpr` support in C23. https://godbolt.org/z/4bThdsqK5

https://github.com/llvm/llvm-project/pull/97846