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

[Aaron Ballman via cfe-commits]

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+/* RUN: %clang_cc1 -std=c89 -fsyntax-only -verify -pedantic -Wno-strict-prototypes %s
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AaronBallman wrote:

Can you also add RUN lines for: `-Wmain-return-type` (enables diagnostic), `-pedantic -Wno-main-return-type` (disables diagnostic), and `` (disables diagnostic)?

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