[llvm] [llvm][ADT] Add `getSingleElement` helper (PR #131508)

[Jakub Kuderski via llvm-commits]

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@@ -1043,6 +1040,14 @@ TEST(STLExtrasTest, getSingleElement) {
   some_namespace::some_struct S;
   S.data = V2;
   EXPECT_EQ(getSingleElement(S), 8);
+
+  // Make sure that we crash on empty or too many elements.
+  SmallVector<int> V4;
+  EXPECT_DEATH(getSingleElement(V4), "expected container with single element");
+  SmallVector<int> V5{12, 13, 14};
+  EXPECT_DEATH(getSingleElement(V5), "expected container with single element");
+  std::list<int> L2;
+  EXPECT_DEATH(getSingleElement(L2), "expected container with single element");
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kuhar wrote:

I think we need to put `#ifndef NDEBUG` around these

https://github.com/llvm/llvm-project/pull/131508