[clang][CodeGen] Fix size calculation in vbptr split memory region in EmitNullBaseClassInitialization (#184558) When splitting memory stores around multiple virtual base pointers (vbptrs) in the Microsoft ABI, the calculation for the size of the memory region after each vbptr was incorrect. The bug/old calculation: SplitAfterSize = LastStoreSize - SplitAfterOffset This subtracts an absolute offset from a relative size, causing incorrect (too small) sizes after the second vbptr. The correct size should be: SplitAfterSize = (LastStoreOffset + LastStoreSize) - SplitAfterOffset Since all store regions extend to the end of the non-virtual portion (NVSize), this patch uses the simplified form: SplitAfterSize = NVSize - SplitAfterOffset The bug causes the assertion failure: "negative store size!" Fixes https://github.com/llvm/llvm-project/issues/42101
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