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

[via cfe-commits]

https://github.com/Sirraide approved this pull request.

LGTM too.

(I admittedly haven’t checked every line of this because there’s just that many of them, but that’s what CI is for; the changes seem fine overall)

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