test(KT-47806): add tests for the uncovered behavior for generic companion function extensions.

compiler/testData/codegen/box/companion/genericLambdaOnStringCompanion.kt

@@ -0,0 +1,10 @@
+// WITH_RUNTIME
+
+fun <T> T.f(E: (y: T) -> String): Boolean = E(this).isEmpty()
+
+fun fu1() = (String).f { v -> "" }
+
+fun box(): String {
+    if (!fu1()) return "Failed: Expect lambda to return empty string"
+    return "OK"
+}