[clang] Add new flag -Wreturn-mismatch (PR #82872)

[Aaron Ballman via cfe-commits]

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+// RUN: %clang_cc1 -Wreturn-type -Wno-return-mismatch -fsyntax-only -verify %s
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AaronBallman wrote:

Because this is the same test code in both files, let's remove this file, add this RUN line to the other file, and rename the other file `return-type-mismatch.h`.

https://github.com/llvm/llvm-project/pull/82872