[clang] [Clang] add ext warning for missing return in 'main' for C89 mode (PR #134617)

[Aaron Ballman via cfe-commits]

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

LGTM aside from an 80-col formatting nit, thank you for the fix!

https://github.com/llvm/llvm-project/pull/134617