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

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

Didn't know it. I'll revert this. 

@ashwinbanwari it looks like you need to remove the diagnostics with `extern "C++" main` and apply it with libc++ and land this finally.

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