[clang] [clang] [modules] Add err_main_in_named_module (PR #146247)

[Mikail Bagishov via cfe-commits]

MikailBag wrote:

It seems that clang trunk [warns](https://godbolt.org/z/Geedq7o46) on `extern "C++" main`, which is now allowed after P3618.

I think it will be helpful for the UX to land this change after that warning is fixed.

https://github.com/llvm/llvm-project/pull/146247