[clang] [Sema][CTAD] Allow user defined conversion for copy-list-initialization (PR #94752)

[Gábor Spaits via cfe-commits]

spaits wrote:

@zyn0217 Thank you for checking my PR. I was able to simplify the `std::initializer_list` definition. I have also created two different "categories" for the test. One test contains almost the exact code as the one in the reported issue with the same class names. The other one is there to test whether we can handle conversions correctly on multiple levels of nesting (one level deep and top level, I have added this since in the code in the issue everything interesting happens on the first level).

https://github.com/llvm/llvm-project/pull/94752