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

compiler/testData/diagnostics/tests/smartCasts/safecalls/extension.fir.kt

@@ -0,0 +1,6 @@
+fun String.foo(arg: Int) = this[arg]
+
+fun calc(x: String?) {
+    // x should be non-null in arguments list
+    x?.foo(x.length - 1)
+}