[libunwind] [libunwind] Fix running tests with MSan (PR #67860)

[Fangrui Song via cfe-commits]

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@@ -43,6 +43,12 @@
   #define LIBUNWIND_AVAIL
 #endif
 
+#if defined(__SANITIZE_MEMORY__) ||                                            \
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MaskRay wrote:

GCC just doesn't support msan :)

https://github.com/llvm/llvm-project/pull/67860