IR: introduce abstract class IrDeclarationBase

The main purpose of this class is to improve performance of IR visitors
and transformers. `IrElementVisitor.visitDeclaration` now takes
IrDeclarationBase as a parameter, and therefore the call to `accept`
there is now a virtual class call, instead of an interface call.

(cherry picked from commit e97d4717fb4c1fe2b6ad8005217150f0e2abe3a7)

backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/lower/CallableReferenceLowering.kt

@@ -71,7 +71,7 @@ internal class CallableReferenceLowering(val context: Context): FileLoweringPass
                 return result
             }
 
-            override fun visitDeclaration(declaration: IrDeclaration): IrStatement {
+            override fun visitDeclaration(declaration: IrDeclarationBase): IrStatement {
                 lateinit var tempGeneratedClasses: MutableList<IrClass>
                 if (declaration is IrClass) {
                     tempGeneratedClasses = generatedClasses

backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/ir/util/IrUtils2.kt

@@ -136,7 +136,7 @@ object SetDeclarationsParentVisitor : IrElementVisitor<Unit, IrDeclarationParent
         }
     }
 
-    override fun visitDeclaration(declaration: IrDeclaration, data: IrDeclarationParent) {
+    override fun visitDeclaration(declaration: IrDeclarationBase, data: IrDeclarationParent) {
         declaration.parent = data
         super.visitDeclaration(declaration, data)
     }