[clang] [Clang] strengthen checks for 'main' function to meet [basic.start.main] p3 requirements (PR #101853)

[via cfe-commits]

https://github.com/Sirraide approved this pull request.

LGTM.

I’ll wait a bit to see if anyone else wants to comment on this since we’ve gone back and forth over a few things w/ this pr, but if not, I’ll merge this sometime later.

https://github.com/llvm/llvm-project/pull/101853