[clang] [clang] Add test for CWG2149 "Brace elision and array length deduction" (PR #90079)

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

Status of 2149 changed just yesterday to say DR per 2024 Tokyo straw poll. That's where the discrepancy between official `cwg_index.html` and CWG GitHub repo comes (that I link in the description).

https://github.com/llvm/llvm-project/pull/90079