[FIR] Implement NULL_FOR_NONNULL_TYPE diagnostics, fix tests

compiler/tests-spec/testData/diagnostics/linked/type-system/introduction-1/p-6/neg/2.1.fir.kt

@@ -4,44 +4,44 @@
 
 // TESTCASE NUMBER: 1
 fun case_1() {
-    val x: Int = <!INITIALIZER_TYPE_MISMATCH!>null<!>
+    val x: Int = <!NULL_FOR_NONNULL_TYPE!>null<!>
 }
 
 // TESTCASE NUMBER: 2
 fun case_2() {
-    val x: Any = <!INITIALIZER_TYPE_MISMATCH!>null<!>
+    val x: Any = <!NULL_FOR_NONNULL_TYPE!>null<!>
 }
 
 // TESTCASE NUMBER: 3
 fun case_3() {
-    val x: Nothing = <!INITIALIZER_TYPE_MISMATCH!>null<!>
+    val x: Nothing = <!NULL_FOR_NONNULL_TYPE!>null<!>
 }
 
 // TESTCASE NUMBER: 4
 fun case_4() {
-    var x: Int = <!INITIALIZER_TYPE_MISMATCH!>null<!>
+    var x: Int = <!NULL_FOR_NONNULL_TYPE!>null<!>
 }
 
 // TESTCASE NUMBER: 5
 fun case_5() {
-    var x: Any = <!INITIALIZER_TYPE_MISMATCH!>null<!>
+    var x: Any = <!NULL_FOR_NONNULL_TYPE!>null<!>
 }
 
 // TESTCASE NUMBER: 6
 fun case_6() {
-    var x: Nothing = <!INITIALIZER_TYPE_MISMATCH!>null<!>
+    var x: Nothing = <!NULL_FOR_NONNULL_TYPE!>null<!>
 }
 
 // TESTCASE NUMBER: 7
 fun case_7() {
     val x: Int
-    x = null
+    x = <!NULL_FOR_NONNULL_TYPE!>null<!>
 }
 
 // TESTCASE NUMBER: 8
 fun case_8() {
     var x: Int = 10
-    x = null
+    x = <!NULL_FOR_NONNULL_TYPE!>null<!>
 }
 
 // TESTCASE NUMBER: 9

compiler/tests-spec/testData/diagnostics/linked/type-system/introduction-1/p-6/neg/2.2.fir.kt

@@ -43,7 +43,7 @@ class Case8 {
 }
 
 // TESTCASE NUMBER: 9
-fun case_9(): Any = <!RETURN_TYPE_MISMATCH!>null<!>
+fun case_9(): Any = <!NULL_FOR_NONNULL_TYPE!>null<!>
 
 // TESTCASE NUMBER: 10
 fun case_10(x: Int, y: Boolean): Any = <!RETURN_TYPE_MISMATCH!>if (y) x else null<!>
@@ -57,10 +57,10 @@ class Case12 {
     var y: Any
     var z: Any
     init {
-        x = null
-        y = null
+        x = <!NULL_FOR_NONNULL_TYPE!>null<!>
+        y = <!NULL_FOR_NONNULL_TYPE!>null<!>
         z = 10
-        z = null
+        z = <!NULL_FOR_NONNULL_TYPE!>null<!>
     }
 }
 

compiler/tests-spec/testData/diagnostics/notLinked/contracts/declarations/contractBuilder/common/neg/4.fir.kt

@@ -6,7 +6,7 @@ import kotlin.contracts.*
 
 // TESTCASE NUMBER: 1
 fun case_1(): Boolean? {
-    contract { <!ERROR_IN_CONTRACT_DESCRIPTION!>returnsNotNull() implies (<!ARGUMENT_TYPE_MISMATCH!>null<!>)<!> }
+    contract { <!ERROR_IN_CONTRACT_DESCRIPTION!>returnsNotNull() implies (<!NULL_FOR_NONNULL_TYPE!>null<!>)<!> }
     return true
 }