[clang] Fix a crash in constant evaluation of ExtVectorElementExprs (PR #136771)

[Erich Keane via cfe-commits]

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@@ -9197,7 +9197,10 @@ bool LValueExprEvaluator::VisitExtVectorElementExpr(
 
   if (Success) {
     Result.setFrom(Info.Ctx, Val);
-    const auto *VT = E->getBase()->getType()->castAs<VectorType>();
+    QualType BaseType = E->getBase()->getType();
+    if (E->isArrow())
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erichkeane wrote:

Ah, huh, I didn't realize that about `MemberExpr`, I guess this IS consistent.  Odd choice that we made...

https://github.com/llvm/llvm-project/pull/136771