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

[Erich Keane via cfe-commits]

erichkeane 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.

Our 'merge' process is a squash & Merge, so it really means "edit the github title and description to be more descriptive, and that is what will be picked up".  So no need to modify the actual commits.

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