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

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

Please make sure the libc++ tests won't get affected.  I am not sure if the modules related tests in libc++ run by default.

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