[CodeGen] Change placeholder from undef to poison (#134731)

Fill default values of a map with `poison` instead of `undef`. There
should be no functional difference as the default values are overridden
later.

Author: pedroclobo

clang/lib/CodeGen/MicrosoftCXXABI.cpp

@@ -370,7 +370,7 @@ class MicrosoftCXXABI : public CGCXXABI {
     MicrosoftVTableContext &VTContext = CGM.getMicrosoftVTableContext();
     unsigned NumEntries = 1 + SrcRD->getNumVBases();
     SmallVector<llvm::Constant *, 4> Map(NumEntries,
-                                         llvm::UndefValue::get(CGM.IntTy));
+                                         llvm::PoisonValue::get(CGM.IntTy));
     Map[0] = llvm::ConstantInt::get(CGM.IntTy, 0);
     bool AnyDifferent = false;
     for (const auto &I : SrcRD->vbases()) {