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

[Aaron Ballman via cfe-commits]

AaronBallman 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 Release note saying that support has been implemented is already there.

Ooh oops, I thought we had this in Clang 18

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