[FIR-TEST] Add new testdata generated after changes in previous commit

compiler/testData/diagnostics/tests/inference/regressions/kt3559.fir.kt

@@ -0,0 +1,10 @@
+// KT-3559 Strange inference failure error message
+
+public inline fun <T:Any, R> let(subj: T?, body: (T) -> R): R? {
+    return if (subj != null) body(subj) else null
+}
+
+
+fun test(s: String?): String? {
+    return let(s) {s} // Reports: "Inference failed. Expected jet.String? but found jet.String?"
+}