Allow bare types on the right-hand side of as/as?/is/!is

compiler/testData/diagnostics/tests/cast/bare/EitherSafeAs.kt

@@ -0,0 +1,16 @@
+trait Either<out A, out B>
+trait Left<out A>: Either<A, Nothing>
+trait Right<out B>: Either<Nothing, B>
+
+class C1(val v1: Int)
+class C2(val v2: Int)
+
+fun _as_left(e: Either<C1, C2>): Any? {
+    val v = e as? Left
+    return v: Left<C1>?
+}
+
+fun _as_right(e: Either<C1, C2>): Any? {
+    val v = e as? Right
+    return v: Right<C2>?
+}