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

[Ashwin Banwari via cfe-commits]

kish1n wrote:

I can't figure out how to make buildbot run the libc++ tests forcefully, so I add a dummy change in libcxx so the tests will run on this PR, and then will remove the change after.

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