[clang] [CIR] Fix uninitialized value which caused MSAN failure (PR #234274)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Erich Keane (erichkeane)

<details>
<summary>Changes</summary>

This failure showed up in the build bots now that we're enabled by default.  We ended up accessing an invalid index while doing label lowering ops.

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Full diff: https://github.com/llvm/llvm-project/pull/234274.diff


1 Files Affected:

- (modified) clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.h (+1-1) 


``````````diff
diff --git a/clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.h b/clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.h
index c71ae0c93f9d4..d3ef1d968f385 100644
--- a/clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.h
+++ b/clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.h
@@ -84,7 +84,7 @@ struct LLVMBlockAddressInfo {
   // name) pair so it can be patched later.
   llvm::DenseMap<mlir::LLVM::BlockAddressOp, cir::BlockAddrInfoAttr>
       unresolvedBlockAddressOp;
-  int32_t blockTagOpIndex;
+  int32_t blockTagOpIndex = 0;
 };
 
 mlir::LogicalResult lowerToConstrainedFPIntrinsic(

``````````

</details>


https://github.com/llvm/llvm-project/pull/234274