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

[Mariya Podchishchaeva via cfe-commits]

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@@ -3599,6 +3599,11 @@ static void expandArray(APValue &Array, unsigned Index) {
   Array.swap(NewValue);
 }
 
+static void expandVector(APValue &Vec, unsigned NumElements) {
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Fznamznon wrote:

Maybe it is worth leaving a comment saying that unlike `expandArray` this function simply expands an indeterminate value to a vector of indeterminate values. It won't preserve anything that is in `Vec` already. Maybe it is also worth adding an assertion that the incoming value is `indeterminate` to avoid misuse.

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