Add checks for const modifier applicability

1. Must be initialized in-place
2. Can not be open/abstract
3. Can not be an override
4. Can not be delegated
5. Initializer must be a compile-time constant
6. No getters
7. `const` is not applicable to vars or locals
8. `const val` should be whether top-level property or object member

compiler/testData/diagnostics/tests/modifiers/const/applicability.kt

@@ -0,0 +1,75 @@
+// !DIAGNOSTICS:-UNUSED_VARIABLE
+
+const val topLevel: Int = 0
+const val topLevelInferred = 1
+<!WRONG_MODIFIER_TARGET!>const<!> var topLeveLVar: Int = 2
+
+private val privateTopLevel = 3
+
+object A {
+    const val inObject: Int = 4
+}
+
+class B(<!CONST_VAL_NOT_TOP_LEVEL_OR_OBJECT!>const<!> val constructor: Int = 5)
+
+abstract class C {
+    <!INCOMPATIBLE_MODIFIERS!>open<!> <!CONST_VAL_NOT_TOP_LEVEL_OR_OBJECT, INCOMPATIBLE_MODIFIERS!>const<!> val x: Int = 6
+
+    <!INCOMPATIBLE_MODIFIERS!>abstract<!> <!CONST_VAL_NOT_TOP_LEVEL_OR_OBJECT, INCOMPATIBLE_MODIFIERS!>const<!> val y: Int = <!ABSTRACT_PROPERTY_WITH_INITIALIZER!>7<!>
+
+    companion object {
+        const val inCompaionObject = 8
+    }
+}
+
+<!ABSTRACT_MEMBER_NOT_IMPLEMENTED!>object D<!> : C() {
+    <!INCOMPATIBLE_MODIFIERS!>override<!> <!INCOMPATIBLE_MODIFIERS!>const<!> val x: Int = 9
+
+    const val inObject = 10
+
+    final const val final = 11
+
+    <!CONST_VAL_WITHOUT_INITIALIZER!>const<!> val withoutInitializer: Int
+
+    init {
+        withoutInitializer = 12
+    }
+}
+
+const val delegated: Int <!CONST_VAL_WITH_DELEGATE!>by Delegate()<!>
+
+
+const val withGetter: Int
+<!CONST_VAL_WITH_GETTER!>get() = 13<!>
+
+fun foo(): Int {
+    <!WRONG_MODIFIER_TARGET!>const<!> val local: Int = 14
+    return 15
+}
+
+enum class MyEnum {
+    A {
+        <!CONST_VAL_NOT_TOP_LEVEL_OR_OBJECT!>const<!> val inEnumEntry = 16
+    };
+    <!CONST_VAL_NOT_TOP_LEVEL_OR_OBJECT!>const<!> val inEnum = 17
+}
+
+class Outer {
+    inner class Inner {
+        object C {
+            const val a = 18
+        }
+    }
+}
+
+const val defaultGetter = 19
+    <!CONST_VAL_WITH_GETTER!>get<!>
+
+const val nonConstInitializer = <!CONST_VAL_WITH_NON_CONST_INITIALIZER!>foo()<!>
+
+// ------------------
+class Delegate {
+    fun get(thisRef: Any?, prop: PropertyMetadata): Int = 1
+
+    fun set(thisRef: Any?, prop: PropertyMetadata, value: Int) = Unit
+}