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

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

@@ -0,0 +1,11 @@
+// KT-9051: Allow smart cast for captured variables if they are not modified
+
+fun bar(z: String?) = z
+
+fun foo(y: String?) {
+    var x: String? = ""
+    if (x != null) {
+        bar(y?.let { x = null; it }).<!INAPPLICABLE_CANDIDATE!>length<!>
+        x.length // Smart cast is not possible
+    }
+}