[clang] [clang][llvm-lit] Rewrite constexpr vectors test to use element access (PR #102757)

[Timm Baeder via cfe-commits]

tbaederr wrote:

can you do `git pull origin main --rebase` and force push the result to your branch, so we can get CI coverage for linux?

https://github.com/llvm/llvm-project/pull/102757