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

[Mariya Podchishchaeva via cfe-commits]

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@@ -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());
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Fznamznon wrote:

The incoming APValue is indeterminate. An indeterminate value is technically not a vector, it is an indeterminate value, so querying a component from it causes failures. In this function we expand it so we can initialize a vector per component.

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