[clang] [Clang][Sema] Fix templated array size calculation. (PR #96464)

[via cfe-commits]

awson wrote:

> It seems reasonable to me, but I want @cor3ntin to review this, he's the one who got his foot stuck in array bounds for a while :D
> 
> However, this patch needs a few changes:
> 
> 1- A better patch title, perhaps improved commit message 2- Release note

1. Partially (patch title) done. After `llvm:main` merge (I've done it to sync release notes) "improved commit message" is a bit problematic, I fear.
2. Done.

https://github.com/llvm/llvm-project/pull/96464