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

[Mital Ashok via cfe-commits]

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

Diagnostic changes are a minor nit, LGTM otherwise

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