K2: Fix false-positive UNSAFE_CALL on safe call with flexible type alias

The idea of the fix is just unwrapping flexible version of type alias,
before creating a not-nullable version for safe-call subject.

Otherwise, it was remained to be JsExpressionResult after applying
makeConeTypeDefinitelyNotNullOrNotNull (and the type is still nullable)

^KT-57190 Fixed

compiler/testData/diagnostics/tests/safeCalls/safeCallOnFlexibleTypeAlias.kt

@@ -0,0 +1,23 @@
+// FIR_IDENTICAL
+// ISSUE: KT-57190
+// FILE: MyFunction.java
+@FunctionalInterface
+public interface MyFunction<T> {
+    void apply(T t);
+}
+
+// FILE: MyPromise.java
+public interface MyPromise<T> {
+    void then(MyFunction<T> done);
+}
+
+// FILE: main.kt
+typealias JsExpressionResult = String?
+
+fun countElementsByXpathAsync(promise: MyPromise<JsExpressionResult>, t: JsExpressionResult) {
+    promise.then {
+        it?.foo() // K1: ok, K2: was UNSAFE_CALL
+    }
+}
+
+fun String.foo() {}