[clang] [Sema] Avoid an undesired pack expansion while transforming PackIndexingType (PR #90195)

[Younan Zhang via cfe-commits]

================
@@ -160,3 +160,22 @@ namespace GH88929 {
     using E = P...[0]; // expected-error {{unknown type name 'P'}} \
                        // expected-error {{expected ';' after alias declaration}}
 }
+
+namespace GH88925 {
+template <typename...> struct S {};
+
+template <int...> struct sequence {};
+
+template <typename... Args, int... indices> auto f(sequence<indices...>) {
+  return S<Args...[indices]...>(); // #use
+}
+
----------------
zyn0217 wrote:

I realized those expressions could be PRValues after an instantiation because in `PackIndexingExpr` we have:
```cpp
if (!isInstantiationDependent())
  setValueKind(getSelectedExpr()->getValueKind());
```

So, I'm going to make it always an LValue iff it's dependent.

https://github.com/llvm/llvm-project/pull/90195