[test] Add minimized test case from SR-7337 (#17524)

Because we've changed how conformances represent associated types
since Swift 4.2 branched, the error described there no longer occurs.
But we still want to make sure we don't regress.

https://bugs.swift.org/browse/SR-7337

Author: jrose-apple

validation-test/Serialization/SR7337.swift

@@ -0,0 +1,20 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-build-swift -emit-module -o %t/Lib.swiftmodule %s -DLIB
+// RUN: %target-build-swift -emit-module -o %t/main.swiftmodule -I %t %s
+
+#if LIB
+
+protocol Proto {}
+
+open class Base<T> {}
+public struct ArbitraryStruct {}
+
+extension Base: Proto where T: Proto {}
+
+#else // LIB
+
+import Lib
+
+final class ConcreteSub: Base<ArbitraryStruct> {}
+
+#endif // LIB