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

[Aaron Ballman via cfe-commits]

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@@ -0,0 +1,17 @@
+// RUN: %clang_cc1 %s -triple i686-pc-win32 -fsyntax-only -verify -fms-compatibility
+// expected-no-diagnostics
+
+typedef __typeof__(sizeof(0)) size_t;
+
+#if defined _MSC_VER && !defined _CRT_USE_BUILTIN_OFFSETOF
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AaronBallman wrote:

We should also have a test for `#ifdef _CRT_USE_BUILTIN_OFFSETOF` to make it clear that we're testing that the macro is predefined.

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