[FIR] Rename FirClassImpl to FirRegularClassImpl

Also rename `buildClassImpl` to `buildRegularClass` and get rid
  of intermediate `AbstractFirRegularClassBuilder`

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/BuilderConfigurator.kt

@@ -32,10 +32,11 @@ object BuilderConfigurator : AbstractBuilderConfigurator<FirTreeBuilder>(FirTree
             fields from klass without listOf("symbol", "resolvePhase", "attributes")
         }
 
-        val regularClassBuilder by builder("AbstractFirRegularClassBuilder") {
+        builder(regularClass) {
             parents += classBuilder
             parents += typeParameterRefsOwnerBuilder
-            fields from regularClass without "attributes"
+            defaultNull("companionObject")
+            openBuilder()
         }
 
         val qualifiedAccessBuilder by builder {
@@ -81,12 +82,6 @@ object BuilderConfigurator : AbstractBuilderConfigurator<FirTreeBuilder>(FirTree
             openBuilder()
         }
 
-        builder(regularClass) {
-            parents += regularClassBuilder
-            defaultNull("companionObject")
-            openBuilder()
-        }
-
         builder(anonymousObject) {
             parents += classBuilder
         }

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/ImplementationConfigurator.kt

@@ -31,12 +31,10 @@ object ImplementationConfigurator : AbstractFirTreeImplementationConfigurator()
         noImpl(declarationStatus)
         noImpl(resolvedDeclarationStatus)
 
-        val regularClassConfig: ImplementationContext.() -> Unit = {
+        impl(regularClass) {
             defaultFalse("hasLazyNestedClassifiers", withGetter = true)
         }
 
-        impl(regularClass, "FirClassImpl", regularClassConfig)
-
         impl(anonymousObject)
 
         impl(typeAlias)