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.

compiler/tests-common/tests/org/jetbrains/kotlin/ir/AbstractIrTextTestCase.kt

@@ -181,7 +181,7 @@ abstract class AbstractIrTextTestCase : AbstractIrGeneratorTestCase() {
         }
 
         @OptIn(ObsoleteDescriptorBasedAPI::class)
-        override fun visitDeclaration(declaration: IrDeclaration) {
+        override fun visitDeclaration(declaration: IrDeclarationBase) {
             if (declaration is IrSymbolOwner) {
                 declaration.symbol.checkBinding("decl", declaration)