[clang] [Clang] fix crash when constexpr evaluation encounters uninitialized GCC vector (PR #180293)

[Shafik Yaghmour via cfe-commits]

================
@@ -3599,6 +3599,13 @@ static void expandArray(APValue &Array, unsigned Index) {
   Array.swap(NewValue);
 }
 
+// Expand an indeterminate vector to materialize all elements.
+static void expandVector(APValue &Vec, unsigned NumElements) {
+  assert(Vec.isIndeterminate());
----------------
shafik wrote:

I am looking at the other expand functions and I am curious why do we want to assert that it has to be indeterminate? Why is that case specifically problematic? Why don't we need to expand non indeterminate vectors?

https://github.com/llvm/llvm-project/pull/180293