[compiler-rt] [TySan] Fix struct access with different bases (PR #120412)

[Florian Hahn via llvm-commits]

================
@@ -131,6 +131,14 @@ static bool isAliasingLegalUp(tysan_type_descriptor *TDA,
           break;
       }
 
+      // This offset can't be negative. Therefore we must be accessing something
+      // partially inside the last type
+      // We shouldn't check this if we are on the first member, Idx will
+      // underflow The first member can be offset in rare cases such as
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fhahn wrote:

```suggestion
      // underflow. The first member can be offset in rare cases such as
```

https://github.com/llvm/llvm-project/pull/120412