Treat constructors as actual only if the 'actual' modifier is present

Exactly as this is done for functions in
FunctionDescriptorResolver.initializeFunctionDescriptorAndExplicitReturnType

 #KT-21906 Fixed

compiler/testData/diagnostics/tests/multiplatform/headerClass/actualClassWithDifferentConstructors.kt

@@ -11,6 +11,7 @@ expect class Bar2()
 expect class Bar3()
 expect class Bar4()
 expect class Bar5()
+expect class Bar6()
 
 // MODULE: m2-jvm(m1-common)
 
@@ -41,5 +42,9 @@ actual class Bar4(val s: String) {
 
 actual class Bar5 {
     actual constructor()
-    constructor(s: String)
-}
+    constructor(<!UNUSED_PARAMETER!>s<!>: String)
+}
+
+class <!ACTUAL_MISSING!>Bar6<!> {
+    actual constructor()
+}

compiler/testData/diagnostics/tests/multiplatform/headerClass/actualClassWithDifferentConstructors.txt

@@ -36,6 +36,13 @@ public final expect class Bar5 {
     public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
 }
 
+public final expect class Bar6 {
+    public constructor Bar6()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
 public final expect class Foo1 {
     public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
     public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
@@ -91,6 +98,13 @@ public final actual class Bar5 {
     public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
 }
 
+public final class Bar6 {
+    public constructor Bar6()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
 public final actual class Foo1 {
     public constructor Foo1(/*0*/ s: kotlin.String)
     public final val s: kotlin.String

compiler/testData/diagnostics/tests/multiplatform/modifierApplicability.kt

@@ -22,7 +22,7 @@ fun foo() {
 // MODULE: m2-jvm
 // FILE: jvm.kt
 
-class Outer actual constructor() {
+class Outer <!ACTUAL_WITHOUT_EXPECT!>actual constructor()<!> {
     actual class <!ACTUAL_WITHOUT_EXPECT!>Nested<!>
 
     <!WRONG_MODIFIER_TARGET!>actual<!> init {}