[clang] [libcxx] [clang] [modules] Add err_main_in_named_module (PR #146635)

[Ashwin Kishin Banwari via cfe-commits]

kish1n wrote:

The only fail is some unrelated test timeout, seems to be flaky. I will remove the added libcxx change now so its ready to merge

https://github.com/llvm/llvm-project/pull/146635