Add regression test for generic records (#19578)

Author: yilinwei

tests/pos-java16+/java-records/FromScala.scala

@@ -41,3 +41,7 @@ object C:
     val l2: Long = r3.l(43L, 44L)
     // supertype
     val isRecord: java.lang.Record = r3
+
+  def useR4: Unit =
+    val r4 = R4(1)
+    val i: Int = r4.t

tests/pos-java16+/java-records/R4.java

@@ -0,0 +1 @@
+public record R4<T>(T t) {}