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

compiler/testData/diagnostics/testsWithStdLib/smartcasts/letUsesOwnReceiver.fir.kt

@@ -0,0 +1,7 @@
+// Based on KT-9033
+fun f(s: String) = s
+
+fun foo(s: String?) {
+    s?.let { f(s) }
+    s?.let { f(it) }
+}