FirCallCompleter: unwrap definitely not-null type properly

#KT-50180 Fixed

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

@@ -0,0 +1,14 @@
+FILE: test.kt
+    public open class Value : R|kotlin/Any| {
+        public constructor(s: R|kotlin/String|): R|Value| {
+            super<R|kotlin/Any|>()
+        }
+
+        public final val s: R|kotlin/String| = R|<local>/s|
+            public get(): R|kotlin/String|
+
+    }
+    public final val generator: R|Generator| = R|/Generator.Generator|()
+        public get(): R|Generator|
+    public final val y: R|Value| = (R|/generator|.R|/Generator.createValue|<R|ft<Value, Value?>|>(String(Omega)) as R|Value|)
+        public get(): R|Value|

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<!>