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

[Akira Hatanaka via cfe-commits]

================
@@ -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())
----------------
ahatanak wrote:

Is `sub-expression` the base expression in this case? For `ExtVectorElementExpr`, the base expression is whatever the LHS of `->` or `.` is and that decision was made a long time ago. I think `MemberExpr`'s base expression can be a pointer type too.

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