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

[via cfe-commits]

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@@ -437,6 +442,16 @@ namespace {
       MostDerivedArraySize = 2;
       MostDerivedPathLength = Entries.size();
     }
+    void addVectorUnchecked(QualType EltTy, uint64_t Size, uint64_t Idx) {
+      Entries.push_back(PathEntry::ArrayIndex(Idx));
+
+      // This is technically a most-derived object, though in practice this
+      // is unlikely to matter.
+      MostDerivedType = EltTy;
+      MostDerivedIsArrayElement = true;
+      MostDerivedArraySize = Size;
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sethp wrote:

I'm not sure about this: it seems to indicate that an expression like `&V[0]` ought to be valid, but it's often not possible to take a pointer to a vector element (i.e. when they're living in a vector register).

That said, I see it's what the `Complex` type is doing, and I whose elements I think are also not individually addressable, so :shrug: 

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