[FIR] Make FirRegularClass.companionObject companionObjectSymbol field

This is needed for two reasons:
1. Before this change companion object appeared in FirRegularClass
  twice: in declarations list and in companionObject field. This may
  trigger twice transform of it
2. It's very hard to implement generation of companion object by plugins
  because if it is part of the tree then generated declaration must be
  registered in FirProvider, which is inconsistent with other generated
  declarations. Replacing FIR with symbol and removing custom logic of
  visiting/transforming companion FIR allows us to just replace companionSymbol
  in FirClass if plugin wants to generate it without any additional work

compiler/fir/java/src/org/jetbrains/kotlin/fir/java/declarations/FirJavaClass.kt

@@ -61,9 +61,6 @@ class FirJavaClass @FirImplementationDetail internal constructor(
 
     override val attributes: FirDeclarationAttributes = FirDeclarationAttributes()
 
-    override val companionObject: FirRegularClass?
-        get() = null
-
     override fun replaceSuperTypeRefs(newSuperTypeRefs: List<FirTypeRef>) {
         superTypeRefs.clear()
         superTypeRefs.addAll(newSuperTypeRefs)
@@ -79,6 +76,11 @@ class FirJavaClass @FirImplementationDetail internal constructor(
 
     override fun replaceControlFlowGraphReference(newControlFlowGraphReference: FirControlFlowGraphReference?) {}
 
+    override val companionObjectSymbol: FirRegularClassSymbol?
+        get() = null
+
+    override fun replaceCompanionObjectSymbol(newCompanionObjectSymbol: FirRegularClassSymbol?) {}
+
     override fun <R, D> acceptChildren(visitor: FirVisitor<R, D>, data: D) {
         declarations.forEach { it.accept(visitor, data) }
         annotations.forEach { it.accept(visitor, data) }
@@ -116,10 +118,6 @@ class FirJavaClass @FirImplementationDetail internal constructor(
         return this
     }
 
-    override fun <D> transformCompanionObject(transformer: FirTransformer<D>, data: D): FirJavaClass {
-        return this
-    }
-
     override fun <D> transformTypeParameters(transformer: FirTransformer<D>, data: D): FirRegularClass {
         typeParameters.transformInplace(transformer, data)
         return this
@@ -166,13 +164,6 @@ internal class FirJavaClassBuilder : FirRegularClassBuilder(), FirAnnotationCont
         )
     }
 
-    @Deprecated("Modification of 'hasLazyNestedClassifiers' has no impact for FirRegularClassImplBuilder", level = DeprecationLevel.HIDDEN)
-    override var companionObject: FirRegularClass?
-        get() = throw IllegalStateException()
-        set(@Suppress("UNUSED_PARAMETER") value) {
-            throw IllegalStateException()
-        }
-
     @Deprecated("Modification of 'origin' has no impact for FirJavaClassBuilder", level = DeprecationLevel.HIDDEN)
     override var origin: FirDeclarationOrigin
         get() = throw IllegalStateException()