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

[Oleksandr T. via cfe-commits]

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

@AaronBallman I've added additonal tests

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