[clang] [clang][ExprConst] allow single element access of vector object to be constant expression (PR #72607)

[Seth Pellegrino via cfe-commits]

sethp wrote:

Ah, sorry: my specific question is whether the APValue::LValuePathEntry ought to grow an understanding of vector types rather than re-using the array machinery, as here. It sounds like arrays express a couple of properties vectors don't, so there's potential for the evaluator to want to distinguish an LValuePath that ends in an array element from one that references a vector component.

That said, I don't have a strong sense of whether it's worth doing. "Sema won't build an AST that the evaluator would need to reject" is a fair way to avoid telling the two apart, but seems much harder to me to demonstrate: "none of the ways we tried to get the evaluator to mistreat a vector as an array" is only the same property if we've managed to test exhaustively. For what my non-expert opinion is worth, something like this seems more robust to me:

```diff
     void addVectorUnchecked(QualType EltTy, uint64_t Size, uint64_t Idx) {
-      Entries.push_back(PathEntry::ArrayIndex(Idx));
+      Entries.push_back(PathEntry::VectorElement(Idx));

-      // This is technically a most-derived object, though in practice this
-      // is unlikely to matter.
       MostDerivedType = EltTy;
-      MostDerivedIsArrayElement = true;
-      MostDerivedArraySize = Size;
```

(assuming that's coherent, it's been a few months)

https://github.com/llvm/llvm-project/pull/72607