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

This patch adds test for
[CWG2149](https://cplusplus.github.io/CWG/issues/2149.html), following
[P3106R1](https://wg21.link/p3106R1) "Clarifying rules for brace elision
in aggregate initialization" and a clarification note on top of it added
on April 2024.

I haven't found a better way to check for equality of values inside
array in 98 mode than to dump AST. I'm open to suggestions there.