[clang] [clang] Predefine `_CRT_USE_BUILTIN_OFFSETOF` in MS-compatible modes (PR #127568)

[Aaron Ballman via cfe-commits]

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+// RUN: %clang_cc1 %s -triple i686-pc-win32 -fsyntax-only -verify -fms-compatibility
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AaronBallman wrote:

We should have a RUN line which is not `-fms-compatibility` to show we don't define it in that case.

https://github.com/llvm/llvm-project/pull/127568