[clang] [C++20][modules] Fix std::initializer_list recognition if it's exported out of a module (PR #118537)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

It looks nice to backport this if we're still in the release circle for 19.x

https://github.com/llvm/llvm-project/pull/118537