[FIR] Implement FUNCTION_EXPECTED

compiler/testData/diagnostics/tests/smartCasts/variables/whileTrue.fir.kt

@@ -0,0 +1,13 @@
+fun x(): Boolean { return true }
+
+public fun foo(pp: Any): Int {
+    var p = pp
+    while(true) {
+        (p as String).length
+        if (x()) break
+        p = 42
+    }
+    // Smart cast is NOT possible here
+    // (we could provide it but p = 42 makes it difficult to understand)
+    return p.<!FUNCTION_EXPECTED!>length<!>()
+}