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

[Dmitriy Chestnykh via cfe-commits]

chestnykh wrote:

> LGTM but please add a release note to clang/docs/ReleaseNotes.rst so users know about the fix as well.
> 
> Once that's added, will you need someone to land the changes on your behalf?

@AaronBallman , yes, i will need this. I'm going to add release notes right now.

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