[clang] [Clang] Fix dependency computation for pack indexing expression (PR #91933)

[via cfe-commits]

================
@@ -377,7 +377,7 @@ ExprDependence clang::computeDependence(PackExpansionExpr *E) {
 ExprDependence clang::computeDependence(PackIndexingExpr *E) {
   ExprDependence D = E->getIndexExpr()->getDependence();
   if (D & ExprDependence::Value)
-    D |= ExprDependence::TypeInstantiation;
+    D |= E->getPackIdExpression()->getDependence() & ExprDependence::Type;
----------------
RungeCC wrote:

The change seems reasonable to me, however with it we need to alter the way `PackIndexingExpr` is handled inside the `getDecltypeForExpr`, since when 1) `E` is not type-dependent (`int...` case) and 2) `index` is instantiation-dependent we shall not directly call `getSelectedExpr`.

https://github.com/llvm/llvm-project/blob/ad1a65fcacda8794e2f1fa3e1dec1c1b7813422c/clang/lib/Sema/SemaType.cpp#L9739-L9748

https://github.com/llvm/llvm-project/blob/ad1a65fcacda8794e2f1fa3e1dec1c1b7813422c/clang/include/clang/AST/ExprCXX.h#L4411-L4415

In short, crash on the following code:

```c++
template<int...args> // notice the "int..." here.
void test(){
  [&]<int idx>(){
    using R = decltype( args...[idx] ) ;
  }.template operator()<0>();
}

void f( ) {
  test<1>();
}

```

https://github.com/llvm/llvm-project/pull/91933