Regular modifier checker implemented (initial version). A set of tests fixed accordingly.

Most of modifier diagnostic is expressed by REDUNDANT_MODIFIER, INCOMPATIBLE_MODIFIERS, REPEATED_MODIFIER, WRONG_MODIFIER_TARGET, WRONG_MODIFIER_PARENT.
A set of modifier diagnostics is not in use now (but not deleted yet).

compiler/testData/diagnostics/tests/sealed/NeverEnum.kt

@@ -1,4 +1,4 @@
-<!SEALED_MODIFIER_IN_ENUM!>sealed<!> enum class SealedEnum {
+<!WRONG_MODIFIER_TARGET!>sealed<!> enum class SealedEnum {
     FIRST, 
     SECOND;
 

compiler/testData/diagnostics/tests/sealed/NeverFinal.kt

@@ -1,3 +1,3 @@
-<!FINAL_MODIFIER_IN_SEALED!>final<!> sealed class Base {
+<!INCOMPATIBLE_MODIFIERS!>final<!> <!INCOMPATIBLE_MODIFIERS!>sealed<!> class Base {
 
 }

compiler/testData/diagnostics/tests/sealed/NeverInterface.kt

@@ -1,3 +1,3 @@
-<!TRAIT_CAN_NOT_BE_SEALED!>sealed<!> interface Base {
+<!WRONG_MODIFIER_TARGET!>sealed<!> interface Base {
 
 }

compiler/testData/diagnostics/tests/sealed/NeverObject.kt

@@ -1,3 +1,3 @@
-<!ILLEGAL_MODIFIER!>sealed<!> object Sealed {
+<!WRONG_MODIFIER_TARGET!>sealed<!> object Sealed {
 
 }

compiler/testData/diagnostics/tests/sealed/NeverOpen.kt

@@ -1,3 +1,3 @@
-<!OPEN_MODIFIER_IN_SEALED!>open<!> sealed class Base {
+<!INCOMPATIBLE_MODIFIERS!>open<!> <!INCOMPATIBLE_MODIFIERS!>sealed<!> class Base {
 
 }

compiler/testData/diagnostics/tests/sealed/RedundantAbstract.kt

@@ -1,3 +1,3 @@
-<!ABSTRACT_MODIFIER_IN_SEALED!>abstract<!> sealed class Base {
+<!REDUNDANT_MODIFIER!>abstract<!> sealed class Base {
 
 }