[FIR] Implement NULL_FOR_NONNULL_TYPE diagnostics, fix tests

compiler/testData/diagnostics/tests/explicitDefinitelyNotNull/simple.fir.kt

@@ -4,7 +4,7 @@ fun <T> foo(x: T, y: T!!): T!! = x ?: y
 
 fun main() {
     foo<String>("", "").length
-    foo<String>("", <!ARGUMENT_TYPE_MISMATCH!>null<!>).length
+    foo<String>("", <!NULL_FOR_NONNULL_TYPE!>null<!>).length
     foo<String?>(null, "")<!UNSAFE_CALL!>.<!>length
     foo<String?>(null, null)<!UNSAFE_CALL!>.<!>length