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

[Jakub Kuderski via llvm-commits]

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@@ -1016,6 +1016,35 @@ TEST(STLExtrasTest, hasSingleElement) {
   EXPECT_FALSE(hasSingleElement(S));
 }
 
+TEST(STLExtrasTest, getSingleElement) {
+  // Note: Asserting behavior of getSingleElement cannot be tested because the
+  // program would crash.
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kuhar wrote:

You can test this with death tests. I'd further argue that if the contract for this function is to crash on ranges with a different number of elements, we should test this. Grep for `EXPECT_DEATH` in this file for other examples.

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