FirCallCompleter: unwrap definitely not-null type properly

#KT-50180 Fixed

compiler/fir/analysis-tests/testData/resolveWithStdlib/problems/definitelyNotNullCast.kt

@@ -0,0 +1,17 @@
+// FILE: Generator.java
+
+import org.jetbrains.annotations.NotNull;
+
+public class Generator {
+    @NotNull
+    public <T extends Value> T createValue(@NotNull String content) {
+        return (T) (new Value(content));
+    }
+}
+
+// FILE: test.kt
+open class Value(val s: String)
+
+val generator = Generator()
+
+val y = generator.createValue("Omega") <!USELESS_CAST!>as Value<!>