Add a regression test for recent regression in pair.

PR #96165 broke code similar to this test, and was subsequently
reverted. Add a test-case, to ensure the problem won't reoccur.

libcxx/test/libcxx/utilities/utility/pairs/pairs.pair/pair.incomplete.verify.cpp

@@ -0,0 +1,15 @@
+// Check that instantiating pair doesn't look up type traits "too early", before
+// the contained types have been completed.
+//
+// This is a regression test, to prevent a reoccurrance of the issue introduced in
+// 5e1de27f680591a870d78e9952b23f76aed7f456.
+
+// expected-no-diagnostics
+#include <utility>
+#include <vector>
+
+struct Test {
+  std::vector<std::pair<int, Test>> v;
+};
+
+std::pair<int, Test> p;